Fix replacement for cgi module

[iainrussell]

This should fix the issues seen with #15.

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 13.90%. Comparing base (65ad776) to head (e366966).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
multiurl/http.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #19      +/-   ##
==========================================
+ Coverage   13.24%   13.90%   +0.65%     
==========================================
  Files          14       14              
  Lines        1034     1043       +9     
==========================================
+ Hits          137      145       +8     
- Misses        897      898       +1     

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[iainrussell]

Resolved in a slightly nicer way by #22.