Unpin dependencies and fix minor issues

[desilinguist]

• Unpin numpy and sphinx in requirements.txt and update SKLL to v4.0.1.
• Update all RSMTool models in test data to avoid warnings from scikit-learn.
• Replace deprecated pandas applymap() method for Python versions > 3.8 to avoid deprecation warnings.
• Update dependencies for ReadTheDocs build.

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (bd2085c) 95.85% compared to head (e3c18b5) 95.83%.

Files	Patch %	Lines
rsmtool/preprocessor.py	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #666      +/-   ##
==========================================
- Coverage   95.85%   95.83%   -0.02%     
==========================================
  Files          32       32              
  Lines        4485     4488       +3     
==========================================
+ Hits         4299     4301       +2     
- Misses        186      187       +1     

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[desilinguist]

We can ignore the change in coverage since this build uses python 3.8 and the decrease is expected.