docs: Add section about updating Python requirements

[john-bodley]

SUMMARY

Adding section in CONTRIBUTING.md regarding updating the Python requirements.

TESTING INSTRUCTIONS

CI.

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15574 (b4a85b7) into master (83be06d) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15574      +/-   ##
==========================================
- Coverage   76.95%   76.94%   -0.01%     
==========================================
  Files         976      976              
  Lines       51290    51290              
  Branches     6907     6907              
==========================================
- Hits        39468    39466       -2     
- Misses      11603    11605       +2     
  Partials      219      219              

Flag	Coverage Δ
hive	81.28% <ø> (ø)
javascript	71.41% <ø> (ø)
mysql	81.54% <ø> (ø)
postgres	81.56% <ø> (ø)
presto	81.26% <ø> (-0.01%)	⬇️
python	82.09% <ø> (-0.01%)	⬇️
sqlite	81.17% <ø> (ø)

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

Impacted Files	Coverage Δ
superset/db_engine_specs/presto.py	89.89% <0.00%> (-0.43%)	⬇️

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[nytai]

@john-bodley is this compatible with changes in #15573