Remove support for Python 2.x.

[dblock]

Description

Delete leftover support for Python 2.

Issues Resolved

Part of https://github.com/opensearch-project/opensearch-py/issues/430 (which still asks for removal of 3.7, so not closing).

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov[bot]]

Codecov Report

Merging #548 (d51a46b) into main (a1f942b) will decrease coverage by 0.10%. The diff coverage is 100.00%.

:exclamation: Current head d51a46b differs from pull request most recent head 9a3d74d. Consider uploading reports for the commit 9a3d74d to get more accurate results

@@            Coverage Diff             @@
##             main     #548      +/-   ##
==========================================
- Coverage   70.66%   70.56%   -0.10%     
==========================================
  Files          83       83              
  Lines        7877     7862      -15     
==========================================
- Hits         5566     5548      -18     
- Misses       2311     2314       +3     

Files	Coverage Δ
opensearchpy/__init__.py	100.00% <100.00%> (+7.14%)	:arrow_up:
opensearchpy/connection/__init__.py	100.00% <100.00%> (+25.00%)	:arrow_up:
opensearchpy/helpers/__init__.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes