add number_of_generators for UniversalPolyRing

[SoongNoonien]

It seems like the poor universal polynomials were overlooked again ;-) I tried to make this somehow in line with the MPoly implementation. In the documentation examples for the MPolys we are testing number_of_generators for equality instead of simply printing the result like in the new examples for UniversalPolyRing. Is there a particular reason for this?

[lgoettgens]

Thanks for your PR. At first glance, this looks good. But since we don't have the compatibility tests with Hecke and Oscar running at the moment (AbstractAlgebra already jumped to the next minor version, but downstream hasn't been adapted), I would like to wait (a few days) until this is working again, just to make sure to not accidentally break something.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.01%. Comparing base (ed58b14) to head (44a38d1).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1795   +/-   ##
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  Coverage   88.01%   88.01%           
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  Files         118      118           
  Lines       30043    30044    +1     
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+ Hits        26442    26443    +1     
  Misses       3601     3601           

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