Add Python 3.9 support

[gliptak]

Signed-off-by: Gábor Lipták gliptak@gmail.com

[andrew-glenn]

@gliptak - Thanks for this PR! For housekeeping purposes, I'm flipping this to draft while you iterate on the Travis testing part. Mark it as ready once that's ironed out! 👍

[gliptak]

mypy might not fully support ```3.9``

https://travis-ci.org/github/aws-quickstart/taskcat/jobs/749308055

taskcat/_cli_modules/upload.py:36: error: syntax error in type comment
Found 1 error in 1 file (checked 37 source files)

python/mypy#9761

[hauntsaninja]

You're using a pretty old version of mypy, if you switch to a recent version that error will go away :-)

[gliptak]

@hauntsaninja same error even after moving to latest pre-commit

https://travis-ci.org/github/aws-quickstart/taskcat/jobs/749501718

[gliptak]

https://github.com/pre-commit/pre-commit.com/pull/453/files

[gliptak]

mypy.....................................................................Failed
- hook id: mypy
- exit code: 2
taskcat/testing/_abstract_test.py:67: error: syntax error in type comment
taskcat/testing/_abstract_test.py:78: error: syntax error in type comment
Found 2 errors in 1 file (checked 44 source files)

Looks valid as per https://mypy.readthedocs.io/en/latest/generics.html