Cleanup unused dependencies and deprecated code

[djhoese]

Removed six and docutils dependencies that aren't used. Removed requirements.txt that isn't used. Remove old test suite code that isn't used. Remove deprecated setuptools things like test_requires.

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

[djhoese]

@adybbroe I also noticed a lot of numpy deprecations. Are you OK if I fix those here or should I make another PR?

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.20%. Comparing base (29663bc) to head (d6b5c58).
Report is 2 commits behind head on main.

Files	Patch %	Lines
pyspectral/solar.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #232      +/-   ##
==========================================
+ Coverage   89.82%   90.20%   +0.37%     
==========================================
  Files          23       23              
  Lines        2535     2521      -14     
==========================================
- Hits         2277     2274       -3     
+ Misses        258      247      -11     

Flag	Coverage Δ
unittests	90.20% <81.25%> (+0.37%)	⬆️

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[adybbroe]

@adybbroe I also noticed a lot of numpy deprecations. Are you OK if I fix those here or should I make another PR?

To me this sounds fine, doing it in this PR. Thanks!

[mraspaud]

LGTM