🗑️ Remove __init__.py to comply with PEP 420

[munechika-koyo]

Summary

Eliminate the init.py file to adhere to PEP 420, allowing for namespace packages without requiring an init.py file.

Why do we need this PR?

In CI with Python 3.12 and later, the deprecated namespace configuration produces the following errors:

======================================================================
ERROR: cherab (unittest.loader._FailedTest.cherab)
----------------------------------------------------------------------
ImportError: Failed to import test module: cherab
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.12.13/x64/lib/python3.12/unittest/loader.py", line 429, in _find_test_path
    package = self._get_module_from_name(name)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.13/x64/lib/python3.12/unittest/loader.py", line 339, in _get_module_from_name
    __import__(name)
  File "/home/runner/work/core/core/cherab/__init__.py", line 19, in <module>
    __import__('pkg_resources').declare_namespace(__name__)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
ModuleNotFoundError: No module named 'pkg_resources'

Note

I also fixed the test script because only python -m unittest couldn't discover the cherab directory.
(Reference: https://docs.python.org/3/library/unittest.html#test-discovery)

[jacklovell]

Looks good. I see in CI there is a warning from setuptools about namespace_packages being deprecated. Can you update setup.py to remove this too and check that it still works OK?

[munechika-koyo]

@jacklovell Thank you for your review!
I updated setup.py as well.