Delegate chunked encoding handling to urllib

[leamingrad]

This MR removes the custom handling of sending chunked HTTP requests from requests to delegate to the equivalent logic in urllib3 (https://github.com/urllib3/urllib3/blob/4325867d1ae0d139a11c8689c2d2a5ba2c666c83/src/urllib3/connectionpool.py#L351).

As far as I can tell this has a couple of advantages:

• Remove code that is duplicated between the two libraries
• Allow fixes for outstanding issues in this area to be made in urllib3 so that users of both libraries benefit (I noticed 404 responses for chunk-encoded requests may not be received #4445 when having a quick trawl of the issues but I guess there are more)
• All requests are now logged by the urllib3 debug logger (which was the issue I was originally trying to diagnose)

[leamingrad]

Quick note on testing that I forgot to put in the description: I haven't added any specific tests to cover this feature, but I think that the code is covered by existing tests from what I can see - if there are extras that would be helpful I am happy to add them.

[codecov-io]

Codecov Report

Merging #4958 into master will decrease coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4958      +/-   ##
==========================================
- Coverage   66.89%   66.66%   -0.24%     
==========================================
  Files          15       15              
  Lines        1577     1554      -23     
==========================================
- Hits         1055     1036      -19     
+ Misses        522      518       -4

Impacted Files	Coverage Δ
requests/adapters.py	68.84% <100%> (-1.43%)	⬇️

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