format_signatures: Fix whitespace issues and add tests

[jbms]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Project coverage is 81.38%. Comparing base (3c8fe16) to head (1b47224).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
sphinx_immaterial/apidoc/format_signatures.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #381      +/-   ##
==========================================
+ Coverage   77.87%   81.38%   +3.50%     
==========================================
  Files          68       68              
  Lines        8645     8674      +29     
==========================================
+ Hits         6732     7059     +327     
+ Misses       1913     1615     -298     

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