fix: Fixed API template issues with new httpx version

[fg-mindee]

This PR introduces the following modifications:

• added version constraint to httpx following the breaking change of https://github.com/encode/httpx/releases/tag/0.20.0
• added instructions to modify the code if httpx needs to be updated

This fixes the failing CI check.
Any feedback is welcome!

[codecov]

Codecov Report

Merging #535 (22a4677) into main (3c7303c) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #535      +/-   ##
==========================================
+ Coverage   95.36%   95.38%   +0.02%     
==========================================
  Files         109      109              
  Lines        4184     4184              
==========================================
+ Hits         3990     3991       +1     
+ Misses        194      193       -1     

Flag	Coverage Δ
unittests	95.38% <ø> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
...dels/detection/differentiable_binarization/base.py	91.82% <0.00%> (+0.62%)	⬆️

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[charlesmindee]

thanks!