Incompatible with Python 3.12 because of pkg_resources use

[StevenMaude]

Traceback (most recent call last):
  File "/tmp/pytest-of-runner/pytest-0/test_packaging_bdist_wheel_whl0/bin/opensafely", line 5, in <module>
    from opensafely import main
  File "/tmp/pytest-of-runner/pytest-0/test_packaging_bdist_wheel_whl0/lib/python3.12/site-packages/opensafely/__init__.py", line 10, in <module>
    import pkg_resources
ModuleNotFoundError: No module named 'pkg_resources'

This doesn't just affect the test because opensafely/__init__.py does the same.

The quick fix is:

easy_install, pkg_resources, setuptools and distutils are no longer provided by default in environments created with venv or bootstrapped with ensurepip, since they are part of the setuptools package. For projects relying on these at runtime, the setuptools project should be declared as a dependency and installed separately (typically, using pip).

https://docs.python.org/3/whatsnew/3.12.html#ensurepip

But a better alternative would be removing the use of pkg_resources, if possible. The use of pkg_resources is deprecated in setuptools.