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]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.13%. Comparing base (6300f6b) to head (fe49a73).
Report is 450 commits behind head on develop.

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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     

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