chore: run nightly tf in CI and fix compatibility with TF 2.9.0

[adriangb]

• Update CI to test on Python 3.9
• Add CI cron job to run tests on tf-nightly / sklearn dev every day
• Fix an edge case failing on TF 2.9.0rc2

[github-actions]

📝 Docs preview for commit 6654ec6 at: https://www.adriangb.com/scikeras/refs/pull/271/merge/

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.27%. Comparing base (7f79748) to head (6654ec6).
Report is 19 commits behind head on master.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #271   +/-   ##
=======================================
  Coverage   98.27%   98.27%           
=======================================
  Files           7        7           
  Lines         752      754    +2     
=======================================
+ Hits          739      741    +2     
  Misses         13       13           

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