Fix issues identified by bugbear report

[ponyisi]

I ran flake8-bugbear (https://github.com/PyCQA/flake8-bugbear) on the frontend code and got:

servicex/app/cache.py:76:25: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/cache.py:88:32: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/datasets.py:47:37: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/datasets.py:52:40: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/datasets.py:95:27: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/datasets.py:119:27: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/main.py:56:31: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:60:32: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:87:25: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:112:25: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:113:22: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:205:25: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:206:37: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/app/transforms.py:209:39: B008 Do not perform function calls in argument defaults.  The call is performed only once at function definition time. All calls to your function will reuse the result of that definition-time function call.  If this is intended, assign the function call to a module-level variable and use that variable as a default value.
servicex/func_adl/func_adl_dataset.py:147:17: B011 Do not call assert False since python -O removes these calls. Instead callers should raise AssertionError().
servicex/func_adl/func_adl_dataset.py:183:21: B011 Do not call assert False since python -O removes these calls. Instead callers should raise AssertionError().
tests/test_query_cache.py:237:9: B017 `assertRaises(Exception)` and `pytest.raises(Exception)` should be considered evil. They can lead to your test passing even if the code being tested is never executed due to a typo. Assert for a more specific exception (builtin or custom), or use `assertRaisesRegex` (if using `assertRaises`), or add the `match` keyword argument (if using `pytest.raises`), or use the context manager form with a target.
tests/test_query_cache.py:238:13: B011 Do not call assert False since python -O removes these calls. Instead callers should raise AssertionError().

All of these seem like reasonable complaints.