Support Python 3.9

[JeanChristopheMorinPerso]

Fixes #828.

Add support for Python 3.9.

I also took the occasion to set the python_requires to explicitly set the supported Python versions and prohibit Python 3.9.0. Additionally I removed the logic around the install requires for Python < 3 to use a more modern and static method.

[codecov-commenter]

Codecov Report

Merging #1080 (388f08f) into master (1ba6d78) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1080   +/-   ##
=======================================
  Coverage   78.87%   78.87%           
=======================================
  Files         113      113           
  Lines        6124     6124           
=======================================
  Hits         4830     4830           
  Misses       1294     1294           

Flag	Coverage Δ
unittests	78.87% <ø> (ø)

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[ssteinbach]

The README.md also needs to be updated (badges and so on). Thanks for doing this clean up!

[JeanChristopheMorinPerso]

@ssteinbach I modified the README as requested. I also tried to see if there was other references to supported Python versions and couldn't find anything outside what I have in this PR. Let me know if there is more places to update.

[meshula]

@ssteinbach Does the latest push meet your request on documentation and badges?

[ssteinbach]

Yup this looks good. I think we can ask the question of whether we can drop the older VFX platform support and add 2021.

[ssteinbach]

Thanks @JeanChristopheMorinPerso!

[JeanChristopheMorinPerso]

I was unsure about the VFX platform version as I wasn't sure if we fully support 2022. For example I don't know if the OTIO UI works with PySide 5.15.x.