fix: Revert tox basepython

[john-bodley]

SUMMARY

Accidentally in #24068 I merged a change which bumped the basepython in the tox.ini whilst testing locally. This PR reverts said mistake. Note that in #23890 we dropped support for Python 3.8 and thus the tox basepython should have also been bumped.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

CI.

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• 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 #24124 (07a4e6b) into master (5159861) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24124      +/-   ##
==========================================
+ Coverage   68.28%   68.30%   +0.02%     
==========================================
  Files        1952     1952              
  Lines       75432    75432              
  Branches     8191     8191              
==========================================
+ Hits        51505    51521      +16     
+ Misses      21823    21807      -16     
  Partials     2104     2104              

Flag	Coverage Δ
hive	53.17% <ø> (ø)
javascript	54.70% <ø> (ø)
mysql	78.94% <ø> (?)
postgres	79.01% <ø> (+<0.01%)	⬆️
presto	53.10% <ø> (ø)
python	82.81% <ø> (+0.04%)	⬆️
sqlite	77.54% <ø> (ø)
unit	53.09% <ø> (ø)

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see 6 files with indirect coverage changes

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