fix: allow read_10x_mtx to read numeric gene IDs

[flying-sheep]

• Closes read_10x_mtx cannot deal with numerical gene symbols #3909
• 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 76.93%. Comparing base (b103908) to head (1777014).
⚠️ Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3932      +/-   ##
==========================================
+ Coverage   76.91%   76.93%   +0.02%     
==========================================
  Files         117      117              
  Lines       12460    12460              
==========================================
+ Hits         9583     9586       +3     
+ Misses       2877     2874       -3     

Flag	Coverage Δ
hatch-test.pre	76.93% <100.00%> (+0.02%)	⬆️

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Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	77.30% <100.00%> (+0.70%)	⬆️