fix: bump httpx version

[chuwt]

the version is different:

This is the code in version 0.23 with name proxies:
https://github.com/encode/httpx/blob/89cdd903b2f339c6ae319773c31b8c9352bd9cd8/httpx/_client.py#L1353

This is the code in version 0.28 with name proxy:
https://github.com/encode/httpx/blob/4fb9528c2f5ac000441c3634d297e77da23067cd/httpx/_client.py#L1364

link to #566

[michaelhly]

@chuwt would you mind fixing:

pyproject.toml changed significantly since poetry.lock was last generated. Run poetry lock [--no-update] to fix the lock file.

[chuwt]

Sure, fixed lock file.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.77%. Comparing base (4b71831) to head (b1e3527).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #568   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files          27       27           
  Lines        2236     2236           
=======================================
  Hits         2052     2052           
  Misses        184      184           

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