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

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.89%. Comparing base (28a1ed4) to head (be820e1).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/readwrite.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3932   +/-   ##
=======================================
  Coverage   76.89%   76.89%           
=======================================
  Files         117      117           
  Lines       12450    12450           
=======================================
  Hits         9574     9574           
  Misses       2876     2876           

Flag	Coverage Δ
hatch-test.pre	76.89% <66.66%> (ø)

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Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	76.59% <66.66%> (ø)