gh-39573: Doctest for defining multivariate polynomial ring over InfinityRing
    
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Fixes #34675

Previously, defining a multivariate polynomial ring over InfinityRing
would incorrectly result in an error. As mentioned in the comments under
the linked issue, this behaviour was fixed in version 10.3 but a doctest
was still needed.

This pull request adds the required doctest, verifying that the
statement `PolynomialRing(InfinityRing,2,'x')` correctly generates a
ring instead of throwing an error.

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preview.

### ⌛ Dependencies

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URL: #39573
Reported by: Caleb Van't Land
Reviewer(s):

Author: Release Manager

src/sage/rings/polynomial/polynomial_ring_constructor.py

@@ -621,6 +621,12 @@ def PolynomialRing(base_ring, *args, **kwds):
         ....:                        '_test_distributivity', '_test_prod'])
         sage: R.<x,y> = PolynomialRing(RIF,2)
         sage: TestSuite(R).run(skip=['_test_elements', '_test_elements_eq_transitive'])
+
+    We verify that multivariate polynomial rings over ``InfinityRing`` from
+    :issue:`34675` are fixed::
+
+        sage: PolynomialRing(InfinityRing, 2, 'x')
+        Multivariate Polynomial Ring in x0, x1 over The Infinity Ring
     """
     from sage.rings.semirings.tropical_semiring import TropicalSemiring
     if base_ring not in Rings() and not isinstance(base_ring, TropicalSemiring):