mongomock needs to declare dependency on setuptools (or stop using it)

[wimglenn]

Mongomock uses setuptools unconditionally. However, setuptools (which provides pkg_resources) is not stdlib, and it is not guaranteed to be there. So it needs to be specified as a dependency, otherwise users can get:

$ python3 -c 'import mongomock'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/.../.venv/lib/python3.9/site-packages/mongomock/__init__.py", line 79, in <module>
    from mongomock.__version__ import __version__
  File "/.../.venv/lib/python3.9/site-packages/mongomock/__version__.py", line 1, in <module>
    import pkg_resources
ModuleNotFoundError: No module named 'pkg_resources'

The current depenency tree is missing setuptools:

$ johnnydep mongomock==4.1.2
name              summary
----------------  -----------------------------------------------------------
mongomock==4.1.2  Fake pymongo stub for testing simple MongoDB-dependent code
├── packaging     Core utilities for Python packages
└── sentinels     Various objects to denote special meanings in python

The alternative would be to stop importing pkg_resources, which is deprecated anyway (#848), and use stdlib importlib.metadata to retrieve the installed version instead.

[stur86]

I support switching to the newer importlib implementation, the deprecation warning messages are quite annoying.