building docs with py3.9

[Borda]

What does this PR do?

bump python version for docs

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--1958.org.readthedocs.build/en/1958/

[codecov]

Codecov Report

Merging #1958 (39ed218) into master (7ed02ab) will not change coverage.
Report is 2 commits behind head on master.
The diff coverage is n/a.

❗ Current head 39ed218 differs from pull request most recent head 034a92e. Consider uploading reports for the commit 034a92e to get more accurate results

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1958   +/-   ##
======================================
  Coverage      85%     85%           
======================================
  Files         265     265           
  Lines       15136   15136           
======================================
  Hits        12878   12878           
  Misses       2258    2258