Fixing dependency on the py module during pip installation

[ptpan]

This pull request adds the py module to the dependency of pymtl3 to fix errors after pip install pymtl3

[codecov]

Codecov Report

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

Comparison is base (0244456) 89.18% compared to head (fde62dd) 88.86%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   89.18%   88.86%   -0.32%     
==========================================
  Files         330      330              
  Lines       30440    30440              
==========================================
- Hits        27147    27052      -95     
- Misses       3293     3388      +95     

Files	Coverage Δ
pymtl3/version.py	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes

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