Fix: removed unecessary warning when a file name comprises multiple dots (closes #3899)

[nicolasrochette]

This PR is to address issue #3899

Minor, but thanks again

• Closes irrelevant warning in sc.readwrite.is_valid_filename #3899
• [Tests][] included or not required because: trivial change
• [Release notes][] not necessary because: too minor

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.26%. Comparing base (4c7c5c0) to head (607d394).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/scanpy/readwrite.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3924   +/-   ##
=======================================
  Coverage   78.26%   78.26%           
=======================================
  Files         117      117           
  Lines       12633    12633           
=======================================
  Hits         9887     9887           
  Misses       2746     2746           

Flag	Coverage Δ
hatch-test.low-vers	77.59% <0.00%> (ø)
hatch-test.pre	77.22% <0.00%> (ø)

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Files with missing lines	Coverage Δ
src/scanpy/readwrite.py	78.95% <0.00%> (ø)