Enhancement proposal: replace use of pkg_resources in _version.py performance reasons

[pi-slh]

pkg_resources can contribute to slow application startup times. Some evaluations of this issue are linked here; pypa/setuptools#926

The functionality used in betterproto of looking up betterproto's own installed version can be replaced by https://docs.python.org/3/library/importlib.metadata.html and the https://github.com/python/importlib_metadata backport.

The enhancement being proposed is to remove the pkg_resources import by replacing pkg_resources.get_distribution("betterproto").version with the equivalent importlib lookup.

[MicaelJarniac]

There's now a deprecation warning about pkg_resources:

venv/lib/python3.9/site-packages/betterproto/__init__.py:39: in <module>
    from ._version import __version__
venv/lib/python3.9/site-packages/betterproto/_version.py:1: in <module>
    from pkg_resources import get_distribution
venv/lib/python3.9/site-packages/pkg_resources/__init__.py:121: in <module>
    warnings.warn("pkg_resources is deprecated as an API", DeprecationWarning)
E   DeprecationWarning: pkg_resources is deprecated as an API