gh-38721: Type of Z/nZ NTL polynomial evaluation should be scalar
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When a polynomial over Z/n implemented in NTL is evaluated, the result
should be in Z/n. Currently it's not; it's still an element of the
polynomial ring. Example:
```
sage: R.<x> = PolynomialRing(Zmod(4), 'x', implementation='NTL')
sage: x.subs(1).parent()
Univariate Polynomial Ring in x over Ring of integers modulo 4 (using
NTL)
```
This patch fixes this behavior.
URL: https://github.com/sagemath/sage/pull/38721
Reported by: Kyle Hofmann
Reviewer(s): Kwankyu Lee
