maint(doc): unpin sphinx version for the docs build

[oscarbenjamin]

The docs build was temporarily pinned due to

sphinx-doc/sphinx#10538

References to other Issues or PRs

See #20623 (comment)

Brief description of what is fixed or changed

Unpin the sphinx version used to build the docs so that the latest release will always be used.

Other comments

Release Notes

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[sympy-bot]

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The docs build was temporarily pinned due to

  https://github.com/sphinx-doc/sphinx/issues/10538

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was changed. Do not include the issue number in the title. -->

#### References to other Issues or PRs
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See https://github.com/sympy/sympy/issues/20623#issuecomment-1161692412

#### Brief description of what is fixed or changed

Unpin the sphinx version used to build the docs so that the latest release will always be used.

#### Other comments


#### Release Notes

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  * Added a new solver for logarithmic equations.

* functions
  * Fixed a bug with log of integers.

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[github-actions]

Benchmark results from GitHub Actions

Lower numbers are good, higher numbers are bad. A ratio less than 1
means a speed up and greater than 1 means a slowdown. Green lines
beginning with + are slowdowns (the PR is slower then master or
master is slower than the previous release). Red lines beginning
with - are speedups.

Significantly changed benchmark results (PR vs master)

       before           after         ratio
     [596257ec]       [126b6194]
+       619±0.9μs       1.26±0.6ms     2.04  polygon.PolygonAttributes.time_perimeter
+        562±40μs       1.21±0.6ms     2.15  polygon.PolygonAttributes.time_sides

Significantly changed benchmark results (master vs previous release)

       before           after         ratio
     [77f1d79c]       [596257ec]
     <sympy-1.10.1^0>                 
+      99.0±0.6ms        179±0.9ms     1.81  sum.TimeSum.time_doit

Full benchmark results can be found as artifacts in GitHub Actions
(click on checks at the top of the PR).