Remove redundant six library

[hugovk]

Botocore supports Python 3.6+ so the six library is no longer needed.

Remove the vendored library and replace its uses with the Python 3 code. In part I used https://github.com/asottile/pyupgrade/ for this.

compat.py can be tidied/refactored further, but this PR is already quite big so that can come later.

[codecov-commenter]

Codecov Report

Merging #2512 (748d8f8) into develop (879726e) will increase coverage by 0.33%. The diff coverage is 98.95%.

@@             Coverage Diff             @@
##           develop    #2512      +/-   ##
===========================================
+ Coverage    95.33%   95.67%   +0.33%     
===========================================
  Files           59       59              
  Lines        11437    11386      -51     
===========================================
- Hits         10904    10893      -11     
+ Misses         533      493      -40     

Impacted Files	Coverage Δ
botocore/exceptions.py	99.51% <ø> (-0.01%)	:arrow_down:
botocore/compat.py	92.14% <97.22%> (+20.01%)	:arrow_up:
botocore/auth.py	97.89% <100.00%> (ø)
botocore/awsrequest.py	98.61% <100.00%> (-0.02%)	:arrow_down:
botocore/configloader.py	100.00% <100.00%> (ø)
botocore/docs/bcdoc/docstringparser.py	99.15% <100.00%> (ø)
botocore/docs/sharedexample.py	98.59% <100.00%> (-0.01%)	:arrow_down:
botocore/endpoint.py	98.02% <100.00%> (-0.02%)	:arrow_down:
botocore/handlers.py	96.34% <100.00%> (+<0.01%)	:arrow_up:
botocore/hooks.py	95.66% <100.00%> (ø)
... and 7 more

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[nateprewitt]

Thanks for the PR, @hugovk! We also have this slated as clean up work we'd like to do. There's still some questions about when/if we want to remove six, so this just comes down to scheduling. I'll reply to the PR when we have a clearer time for merging a change like this.

[hugovk]

This has got a bit merge conflicty, let's close and redo when the time is right 👍