Add mention of Python 3.9 in the trove classifier list

[schlenk]

Just adding 3.9 to the supported list.

• Any changes relevant to users are recorded in the CHANGELOG.md.
• The documentation has been updated, if necessary.
• New code is annotated.
• Changes are covered by tests.

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[codecov-commenter]

Codecov Report

Merging #787 (3278a12) into master (b7f118c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #787   +/-   ##
=======================================
  Coverage   63.69%   63.69%           
=======================================
  Files          64       64           
  Lines       11843    11843           
  Branches     2095     2095           
=======================================
  Hits         7543     7543           
  Misses       3704     3704           
  Partials      596      596           

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