Remove httpx pin, fix tests, & test plugins individually

[simonbohnen]

This PR removes the pin on httpx, which was causing issues with newer versions of starlette which require httpx>=0.22.0.
See conda-forge/quetz-feedstock#24.

• Fixed the tests which required changes to work with httpx>=0.22.0
• Changed plugin testing to not abort when tests have failed for one plugin

[codecov-commenter]

Codecov Report

Base: 82.57% // Head: 81.69% // Decreases project coverage by -0.87% ⚠️

Coverage data is based on head (64b7659) compared to base (ae38be7).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #574      +/-   ##
==========================================
- Coverage   82.57%   81.69%   -0.88%     
==========================================
  Files          78       78              
  Lines        6059     6059              
==========================================
- Hits         5003     4950      -53     
- Misses       1056     1109      +53     

Impacted Files	Coverage Δ
quetz/pkgstores.py	42.45% <0.00%> (-11.60%)	⬇️

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[janjagusch]

Thank you!