add eval-type-backport for docs built with python 3.9

[mattseddon]

build docs is currently failing for all platforms in the CI against Python 3.9. This PR follows the provided error message.

example: https://github.com/iterative/datachain/actions/runs/13126453256/job/36623622317

[cloudflare-workers-and-pages]

Deploying datachain-documentation with    Cloudflare Pages

Latest commit:	c68701c
Status:	✅  Deploy successful!
Preview URL:	https://09895c0b.datachain-documentation.pages.dev
Branch Preview URL:	https://add-eval-type-backport.datachain-documentation.pages.dev

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.74%. Comparing base (cc91ed9) to head (c68701c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #895      +/-   ##
==========================================
+ Coverage   87.72%   87.74%   +0.01%     
==========================================
  Files         129      129              
  Lines       11475    11473       -2     
  Branches     1545     1545              
==========================================
  Hits        10067    10067              
+ Misses       1020     1018       -2     
  Partials      388      388              

Flag	Coverage Δ
datachain	87.66% <ø> (+0.01%)	⬆️

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