Bump minimum version of Python to 3.9 and httpx-ws to 0.7.0

[jacobtomlinson]

In #543 we are seeing an issue with the websocket ping conflicting with our interactions with the websocket. It looks like it was fixed upstream in frankie567/httpx-ws#89 and released in httpx-ws version 0.7.0.

This PR bumps our minimum version to ensure we get that fix.

This version of httpx-ws doesn't support Python 3.8 so this feels like a good opportunity to bump out minimum Python version to 3.9.

Closes #543

[jacobtomlinson]

Looks like this change didn't trigger CI correctly so I've tweaked the GitHub Actions workflow to trigger properly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.03%. Comparing base (87063fc) to head (f4c8014).
Report is 155 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #546      +/-   ##
==========================================
+ Coverage   94.61%   95.03%   +0.41%     
==========================================
  Files          29       30       +1     
  Lines        3141     4065     +924     
==========================================
+ Hits         2972     3863     +891     
- Misses        169      202      +33     

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[jacobtomlinson]

It looks like httpx-ws 0.7.0 dropped support for Python 3.8. There are also other 3.8 related failures popping up so it feels like now is to good time for us to drop 3.8 too.