Remove pkg_resources usage in config handling

[djhoese]

pkg_resources is deprecated. This removes the usage while also moving the access of the PSP_CONFIG_FILE environment variable from an import-time operation to a runtime operation inside the config function call.

• Closes #xxxx
• Tests added
• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (6f3b523) 89.96% compared to head (77e3a31) 90.03%.

❗ Current head 77e3a31 differs from pull request most recent head 14f6ec1. Consider uploading reports for the commit 14f6ec1 to get more accurate results

Files	Patch %	Lines
pyspectral/config.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
+ Coverage   89.96%   90.03%   +0.07%     
==========================================
  Files          23       23              
  Lines        2580     2580              
==========================================
+ Hits         2321     2323       +2     
+ Misses        259      257       -2     

Flag	Coverage Δ
unittests	90.03% <87.50%> (+0.07%)	⬆️

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[adybbroe]

LGTM Thanks for this!

[djhoese]

I made one more small update, but github seems to be struggling showing this new commit. Will merge after it is recognized and passes tests.