DOC: Link to Python 3 versions of official Python docs

[jschendel]

• passes git diff upstream/master -u -- "*.py" | flake8 --diff

Went through and made sure that all links to the official Python docs start with https://docs.python.org/3/ to ensure that the links go to the most recent stable version of Python 3.

[jreback]

might be worthwhile to add a lint rule that rejects python links that are not 3?

[codecov]

Codecov Report

Merging #18962 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #18962   +/-   ##
=======================================
  Coverage   91.59%   91.59%           
=======================================
  Files         150      150           
  Lines       48924    48924           
=======================================
  Hits        44813    44813           
  Misses       4111     4111

Flag	Coverage Δ
#multiple	89.95% <ø> (ø)	⬆️
#single	41.16% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/computation/eval.py	97.11% <ø> (ø)	⬆️
pandas/core/indexes/datetimelike.py	97.19% <ø> (ø)	⬆️

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[jreback]

thanks!