test-requirements.txt: remove pinning from cython

since we want to support python 3.12, we need to remove
this pinning, cause those versions are casueing the cython
related unittets to fail:
```
 ImportError while importing test module '/project/tests/unit/cython/test_bytesio.py'.
  Hint: make sure your test modules/packages have valid Python names.
  Traceback:
  /opt/python/cp312-cp312/lib/python3.12/importlib/__init__.py:90: in import_module
      return _bootstrap._gcd_import(name[level:], package, level)
  /project/tests/unit/cython/test_bytesio.py:16: in <module>
      bytesio_testhelper = cyimport('tests.unit.cython.bytesio_testhelper')
  /project/tests/unit/cython/utils.py:29: in cyimport
      import pyximport
  ../venv/lib/python3.12/site-packages/pyximport/__init__.py:1: in <module>
      from .pyximport import *
  ../venv/lib/python3.12/site-packages/pyximport/pyximport.py:51: in <module>
      import imp
  E   ModuleNotFoundError: No module named 'imp'
```

Author: fruch

test-requirements.txt

@@ -10,7 +10,7 @@ twisted[tls]==19.2.1; python_version < '3.5'
 gevent>=1.0; platform_machine != 'i686' and platform_machine != 'win32'
 gevent==20.5.0; platform_machine == 'i686' or platform_machine == 'win32'
 eventlet>=0.33.3
-cython>=0.20,<0.30
+cython
 packaging
 futurist; python_version >= '3.7'
 asynctest; python_version >= '3.5'