Add Python 3.7 Trove classifier

[hugovk]

Describe your changes

Memray still supports Python 3.7, add the missing classifier for PyPI.

Testing performed

None.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.92%. Comparing base (5d9e04d) to head (c8c4ccf).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
- Coverage   93.03%   92.92%   -0.12%     
==========================================
  Files          94       94              
  Lines       11445    11445              
  Branches     2114     2114              
==========================================
- Hits        10648    10635      -13     
- Misses        797      810      +13     

Flag	Coverage Δ
cpp	92.92% <ø> (-0.12%)	⬇️
python_and_cython	92.92% <ø> (-0.12%)	⬇️

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[pablogsal]

Thanks a lot @hugovk ❤️