Fix DisallowedModelAdminLookup in SignalInfoModelAdmin, too.

[jedie]

Reported here:
#135 (comment)

Fix some code style errors in changed files, too ;)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.00000% with 5 lines in your changes missing coverage. Please review.

Project coverage is 67.95%. Comparing base (fa4c3f3) to head (39d87b8).
Report is 6 commits behind head on main.

Files	Patch %	Lines
huey_monitor/admin.py	73.68%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #140      +/-   ##
==========================================
+ Coverage   67.79%   67.95%   +0.16%     
==========================================
  Files          36       36              
  Lines         739      749      +10     
  Branches       88       88              
==========================================
+ Hits          501      509       +8     
- Misses        238      240       +2     

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[cnschn]

LGTM