Add official warnings for the removal of six from vendoring

[nateprewitt]

With the release of Botocore 1.11.0 in 2018, the botocore.vendored module was deprecated and a majority of the contents were removed. The remaining pieces for Requests have been left along with a copy of six. The six module was deprecated in 2021 when Python 2 support was dropped but left for backwards compatibility.

Now that botocore hasn't supported Python 2.7 for 4 years, we intend to remove this from the library in a future release. In the interim, warnings will be raised when direct imports are made against either the compat or vendor instances of six.

This includes both of the below syntaxes:

from botocore.compat import six

import botocore.vendored.six

Customers who still need six should add it to their direct dependencies to receive the latest updates.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.13%. Comparing base (6300f6b) to head (fe49a73).
⚠️ Report is 1259 commits behind head on develop.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3504      +/-   ##
===========================================
+ Coverage    92.94%   93.13%   +0.19%     
===========================================
  Files           66       67       +1     
  Lines        14956    15270     +314     
===========================================
+ Hits         13901    14222     +321     
+ Misses        1055     1048       -7     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[a-detiste]

Thank you for doing this @nateprewitt

I was planning to propose a more intrusive patch on Debian side something of the likes of

-from botocore.vendored import six
+try:
+     import six
+except ImportError:
+     pass

+ setting a loose Recommends: python3-six dependency in the package metadata,
because we are very far in the six removal (we patch the biggest culprit dateutil);
but now I will first wait to see how this very nice PR resolves.

https://wiki.debian.org/Python3-six-removal

Greetings