Fix holonomic

[iammosespaulr]

References to other Issues or PRs

Same as #19236 but for the 1.6 branch.

Brief description of what is fixed or changed

Fixes a cache mismatch due to usingis instead of == in holonomic.py

Other comments

Release Notes

• holonomic
  • A cache mismatch error was fixed.

[sympy-bot]

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• holonomic
  • A cache mismatch error was fixed. (#19248 by @iammosespaulr and @smichr)

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#### References to other Issues or PRs
<!-- If this pull request fixes an issue, write "Fixes #NNNN" in that exact
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Same as #19236 but for the 1.6 branch.

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

Fixes a cache mismatch due to using`is` instead of `==` in `holonomic.py`

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
* holonomic
  * A cache mismatch error was fixed.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #19248 into 1.6 will increase coverage by 0.006%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##               1.6    #19248       +/-   ##
=============================================
+ Coverage   75.650%   75.657%   +0.006%     
=============================================
  Files          651       651               
  Lines       169488    169565       +77     
  Branches     40021     40056       +35     
=============================================
+ Hits        128219    128289       +70     
- Misses       35652     35665       +13     
+ Partials      5617      5611        -6     

[smichr]

@oscarbenjamin , shall I leave this unmerged so you can do all the passing 1.6 milestone PRs at once?