Remove references to retired Python 2

[greg-db]

Remove classifier and workflow settings for Python 2.

[codecov]

Codecov Report

Merging #481 (337f863) into main (487793d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #481   +/-   ##
=======================================
  Coverage   63.69%   63.69%           
=======================================
  Files          31       31           
  Lines       52609    52609           
  Branches     3841     3841           
=======================================
  Hits        33510    33510           
  Misses      19064    19064           
  Partials       35       35           

Flag	Coverage Δ
unit	63.69% <ø> (ø)

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[hugovk]

Also remove pypy-2.7 from

dropbox-sdk-python/.github/workflows/ci.yml

Line 14 in 337f863

python-version: [3.5, 3.6, 3.7, 3.8, pypy-2.7, pypy-3.7]

and py27 from tox.ini