Enable Python 3.9

[anargyri]

Description

• Modify setup.py to allow Python 3.9.
• Edit documentation accordingly.

Related Issues

Checklist:

• I have followed the contribution guidelines and code style for this project.
• I have added tests covering my contributions.
• I have updated the documentation accordingly.
• This PR is being made to staging branch and not to main branch.

[miguelgfierro]

Cool

[codecov-commenter]

Codecov Report

Merging #1626 (091b7a9) into staging (b37a577) will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           staging    #1626      +/-   ##
===========================================
+ Coverage    57.00%   57.01%   +0.01%     
===========================================
  Files           88       88              
  Lines         8956     8956              
===========================================
+ Hits          5105     5106       +1     
+ Misses        3851     3850       -1     

Flag	Coverage Δ
pr-gate	57.01% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
recommenders/models/sasrec/sampler.py	91.66% <0.00%> (+2.08%)	⬆️

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