gh-35277: `sage.rings.polynomial.polynomial_ring[_constructor]`: Handle missing implementation modules
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### ๐ Description
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We modify the `__init__` methods of univariate polynomial implementation
classes so that they all take an `implementation` keyword (and pass it
on to their `super`).
We guard the imports of implementation classes in the `PolynomialRing`
constructor with `try... except ImportError` and fall back to
alternative implementations unless a specific implementation was
requested.
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This is part of:
- #29705
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### โ Dependencies
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URL: https://github.com/sagemath/sage/pull/35277
Reported by: Matthias Kรถppe
Reviewer(s): Marc Mezzarobba
