Migrate from pkg_resources to importlib-based solutions

[stevepiercy]

After following Install icalendar Manually, Building the documentation, and attempting to build the docs, I get the same error as in #621:

$ make html                
sphinx-build -b html -d _build/doctrees   . _build/html
Running Sphinx v7.3.7

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File "/Users/stevepiercy/projects/icalendar/env/lib/python3.12/site-packages/sphinx/config.py", line 509, in eval_config_file
    exec(code, namespace)  # NoQA: S102
    ^^^^^^^^^^^^^^^^^^^^^
  File "/Users/stevepiercy/projects/icalendar/docs/conf.py", line 2, in <module>
    import pkg_resources
ModuleNotFoundError: No module named 'pkg_resources'

make: *** [html] Error 2

Per https://setuptools.pypa.io/en/latest/pkg_resources.html, pkg_resources is deprecated and should be replaced with other tools in the standard library.

Attention

Use of pkg_resources is deprecated in favor of importlib.resources, importlib.metadata and their backports (importlib_resources, importlib_metadata). Some useful APIs are also provided by packaging (e.g. requirements and version parsing). Users should refrain from new usage of pkg_resources and should work to port to importlib-based solutions.

Some examples of how to do that may be found in the Pyramid repo's PRs.

• In docs/conf.py, replace pkg_resources.get_distribution with importlib.metadata. Allow docs to build on Python 3.12 by replacing pkg_resources with … #636
• In bootstrap.py, there are some conditional imports of both pkg_resources and setuptools. I'm not sure what to do with these.