Fix format_signatures to work with Sphinx < 7.3

[jbms]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.19%. Comparing base (162f262) to head (60b0c02).
Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
sphinx_immaterial/apidoc/format_signatures.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #373      +/-   ##
==========================================
- Coverage   67.22%   67.19%   -0.03%     
==========================================
  Files          68       68              
  Lines        8512     8515       +3     
==========================================
  Hits         5722     5722              
- Misses       2790     2793       +3     

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