Handle 100 Continue in urllib3>=2.

[tjni]

This maintains compatibility from urllib3 1.25.4 to urllib3 2.0.3 at least by moving the code in _send_request to request. In urllib3>=2, the _send_request function of http.client.HTTPConnection is skipped in favor of a custom implementation.

Related to #2926.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (59edb73) 93.36% compared to head (560e26f) 93.36%.

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Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2984   +/-   ##
========================================
  Coverage    93.36%   93.36%           
========================================
  Files           65       65           
  Lines        13841    13841           
========================================
  Hits         12923    12923           
  Misses         918      918           

Files Changed	Coverage Δ
botocore/awsrequest.py	98.55% <100.00%> (ø)

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[nateprewitt]

Hi @tjni, thanks for this PR! We're working on the fix internally at the moment but this appears to be on the right track. We will be moving to the request wrapper interface provided in CPython's HTTPConnection class but the way urllib3 wrote this creates some performance issues when the body is dropped from a non-100 response.

I'll have a patch to put up once we've resolved those issues. I can either attempt to rebase my changes onto this branch or will post a new PR. Please let me know if you have a preference.

[tjni]

No preference, except that you do whatever is easiest for you. Thank you for taking a look.

[nateprewitt]

Resolving in favor of #2990. Thanks again, @tjni!