raise error if seq change is detected in uniq

[smichr]

References to other Issues or PRs

fixes #18832

Brief description of what is fixed or changed

When easily detectable, Python raises an error when an item being iterated over is changed and would thus cause the iteration to fail.

When iterating over the unique elements of an iterable as detected by uniq, changing the iterable during iteration will cause a failure in uniq to detect the unique elements. It is not clear that uniq will do this so an error is now raised in the cases when this is easy to detect and a note is added to the docstring.

Other comments

Release Notes

• utilities
  • iterables - uniq will now raise a RuntimeError if a size change of the sequence is detected

[sympy-bot]

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• utilities
  • iterables - uniq will now raise a RuntimeError if a size change of the sequence is detected (#18835 by @smichr)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.6.

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#### References to other Issues or PRs
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fixes #18832 

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

When easily detectable, [Python raises an error](https://stackoverflow.com/a/50167042/1089161) when an item being iterated over is changed and would thus cause the iteration to fail.

When iterating over the unique elements of an iterable as detected by `uniq`, changing the iterable during iteration will cause a failure in `uniq` to detect the unique elements. It is not clear that `uniq` will do this so an error is now raised in the cases when this is easy to detect and a note is added to the docstring.

#### Other comments


#### Release Notes

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<!-- BEGIN RELEASE NOTES -->
* utilities
    * iterables - `uniq` will now raise a RuntimeError if a size change of the sequence is detected
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #18835 into master will increase coverage by 0.004%.
The diff coverage is 100.000%.

@@              Coverage Diff              @@
##            master    #18835       +/-   ##
=============================================
+ Coverage   75.653%   75.657%   +0.004%     
=============================================
  Files          647       647               
  Lines       168516    168526       +10     
  Branches     39706     39708        +2     
=============================================
+ Hits        127488    127503       +15     
+ Misses       35474     35467        -7     
- Partials      5554      5556        +2     

[smichr]

@oscarbenjamin , are you ok with this addition?

[oscarbenjamin]

It seems unnecessary to me but I don't object to it. I think this is fine to merge