Fix mypy errors

[staticdev]

First step to provide type annotations support #152 (in order to do that, python 3.5 need to be dropped)

Current errors:

src/pytest_mock/plugin.py:5: error: Skipping analyzing 'pytest': found module but no type hints or library stubs
src/pytest_mock/plugin.py:5: note: See https://mypy.readthedocs.io/en/latest/running_mypy.html#missing-imports
src/pytest_mock/plugin.py:7: error: Skipping analyzing 'pytest_mock._version': found module but no type hints or library stubs
src/pytest_mock/plugin.py:210: error: Need type annotation for '_mock_module_originals' (hint: "_mock_module_originals: Dict[<type>, <type>] = ...")
src/pytest_mock/plugin.py:238: error: Assignment to variable 'e' outside except: block
Found 4 errors in 1 file (checked 1 source file)

[jtpavlock]

I think it's worth noting that pytest uses type annotations as of 6.0. This would be nice to also implement here, but I am also confused by the requirement of dropping 3.5, since type annotations are supported in 3.5

[staticdev]

@jtpavlock you are correct, it seems to have some workarounds regarding 3.5, I will try to make it compatible based on pytest 6.0 configs.