Add MANIFEST.in to include requirements.txt in sdist

[mashehu]

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Summary

I am trying to add this to bioconda (see bioconda/bioconda-recipes#64333), but it currently fails to build there, because requirements.txt is not in the sdist, but setup.py requires it.

This change makes sure it gets bundled with the rest of the files

Related Issue/Ticket

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.40%. Comparing base (93b2d4b) to head (ce7e6ed).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #966   +/-   ##
=======================================
  Coverage   55.40%   55.40%           
=======================================
  Files          33       33           
  Lines        3061     3061           
=======================================
  Hits         1696     1696           
  Misses       1365     1365           

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