Fix mask_obs parameter resulting in underlying data being modified

[vishnumenon]

• Closes mask_obs parameter modifies obs column when calling scatterplot #3400
• Tests included or not required because:

• Release notes not necessary because:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.18%. Comparing base (7baadb7) to head (de49c7c).
Report is 19 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3496      +/-   ##
==========================================
- Coverage   75.53%   75.18%   -0.36%     
==========================================
  Files         113      113              
  Lines       13252    13253       +1     
==========================================
- Hits        10010     9964      -46     
- Misses       3242     3289      +47     

Files with missing lines	Coverage Δ
src/scanpy/plotting/_tools/scatterplots.py	84.51% <100.00%> (+0.03%)	⬆️

... and 1 file with indirect coverage changes

[flying-sheep]

Thanks!