Start using SSLContext.minimum|maximum_version when available

[sethmlarson]

Closes #2110
Closes #2373

• Adding minimum_version and maximum_version converters to the PyOpenSSL and SecureTransport SSLContext implementations
• Fixing the test suite for the new deprecation message
• Adding documentation about ssl_minimum_version and ssl_maximum_version, removing documentation about ssl_version.
• Creating an issue about deprecating ssl_version once Python 3.6 support is dropped. Right now we're in a soft deprecation as if we were emitting deprecation warnings now it'd make life hard for any library supporting Python 3.6+ and using the option.

[codecov]

Codecov Report

Merging #2220 (f2f3c3e) into main (c674f48) will decrease coverage by 0.72%.
The diff coverage is 97.14%.

❗ Current head f2f3c3e differs from pull request most recent head 9845c2e. Consider uploading reports for the commit 9845c2e to get more accurate results

@@             Coverage Diff             @@
##              main    #2220      +/-   ##
===========================================
- Coverage   100.00%   99.27%   -0.73%     
===========================================
  Files           25       25              
  Lines         2466     2484      +18     
===========================================
  Hits          2466     2466              
- Misses           0       18      +18     

Impacted Files	Coverage Δ
src/urllib3/exceptions.py	100.00% <ø> (ø)
src/urllib3/util/timeout.py	100.00% <ø> (ø)
src/urllib3/response.py	99.26% <83.33%> (-0.74%)	⬇️
src/urllib3/connection.py	100.00% <100.00%> (ø)
src/urllib3/connectionpool.py	100.00% <100.00%> (ø)
src/urllib3/contrib/socks.py	81.08% <100.00%> (-18.92%)	⬇️
src/urllib3/poolmanager.py	100.00% <100.00%> (ø)
src/urllib3/util/request.py	100.00% <100.00%> (ø)
src/urllib3/util/retry.py	100.00% <100.00%> (ø)
src/urllib3/util/ssl_.py	96.39% <100.00%> (-3.61%)	⬇️
... and 4 more

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[pquentin]

You can now remove the Python 3.6 specific code! This will make this easier

[sethmlarson]

Definitely! I have some local changes that rebased this on main without the Python 3.6 fallbacks but still have tests to work though.

[sethmlarson]

I think this is ready for review functionality-wise! For future reviewers please rebase merge this PR as it contains multiple features.

[sethmlarson]

And now the feature is documented too, ready for a full review.

[pquentin]

Thanks! This is great. I've only looked at the code (not the tests or docs) on my phone, hopefully this initial review is still useful.

There are two legitimate coverage issues, can you please take a look?

[pquentin]

Thanks! Feel free to merge after looking at my only comment.