Add Python 3.9 support

[ThePumpingLemma]

Add Python 3.9 support

Description

Add Python 3.9 support

Related Issue

#745

Motivation and Context

Add Python 3.9 support

How Has This Been Tested?

Added Python 3.9 to test matrix

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have run the make format command to format my code
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have added my name and/or github handle to AUTHORS.rst
• I have added my change to CHANGES.rst
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #744 (93c9dfb) into master (e854c48) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #744   +/-   ##
=======================================
  Coverage   97.44%   97.44%           
=======================================
  Files          18       18           
  Lines         980      980           
  Branches      148      148           
=======================================
  Hits          955      955           
  Misses         12       12           
  Partials       13       13           

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