Add PolynomialMod for an integer modulus

[tombouctou]

PolynomialMod is not defined in Mathics3, so even PolynomialMod[3 x ^ 2 + 2 x + 1, 2] stays unevaluated. This adds it for the common case of a positive integer modulus.

What it does. PolynomialMod[poly, m] with a positive integer m expands poly (with the existing expand_polynomial) and reduces each integer coefficient to 0..m-1. As in WMA, parts that are not polynomials, such as f[1] or Sin[x], are treated as variables:

PolynomialMod[3 x ^ 3 + 21 x ^ 2 - 7 x + 55, 9]     (* 1 + 2 x + 3 x ^ 2 + 3 x ^ 3 *)
PolynomialMod[1234 (x + y) ^ 7 - (x - y) ^ 5, 7]    (* 6 x ^ 5 + 2 x ^ 7 + 5 x ^ 4 y + ... + 2 y ^ 7 *)
PolynomialMod[3 f[1] + 2 f[2] - f[3], 2]           (* f[1] + f[3] *)

Code that works with formal sums mod 2 relies on this, e.g. PolynomialMod[del /@ x, 2] where the terms are words like w[3, 1, 1].

What it does not do yet. It leaves these unevaluated rather than guess at WMA's conventions:

• coefficients that are Rational, Real or Complex;
• a modulus that is a polynomial, or a list of moduli;
• a non-positive modulus, and a list as poly.

Tests. The doc examples above, plus test_polynomialmod in test/builtin/numbers/test_algebra.py, which covers negative coefficients, terms that cancel to 0, non-polynomial parts, and each case left unevaluated. pytest and the doctests give the same results as on master. The only failures are environmental ones that also fail on master: XML/ZIP/JPEG FileFormat, and the CLI tests that need mathics on the PATH. mypy is clean on both changed modules.

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