Split sync and async HTTPX handlers

[seowalex]

In #943 (comment), sending sync requests in async contexts was accidentally broken. As #957 and #958 correctly point out, this is because the future's result was not awaited.

Instead of creating a separate event loop as #957 and #958 do, I have opted to properly separate the sync and async HTTPX handlers, as the only reason async functions were being run in a sync context was to reuse the functions.

To do this, I have pushed the logic to convert the byte stream (which is the only truly async function) up into the respective sync/async handlers, which allows the other functions to be sync (and thus callable from the respective handlers without additional work).

I have also added a test for this regression and removed the extraneous pytest.mark.online markers.

[seowalex]

Tagging @jairhenrique given you reviewed my previous PR, I think this is a fairly important bugfix.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.32%. Comparing base (2c4df79) to head (1ef0356).
⚠️ Report is 22 commits behind head on master.
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #965      +/-   ##
==========================================
+ Coverage   92.23%   92.32%   +0.08%     
==========================================
  Files          27       27              
  Lines        1868     1863       -5     
  Branches      249      248       -1     
==========================================
- Hits         1723     1720       -3     
+ Misses         97       94       -3     
- Partials       48       49       +1     

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