Always return int from Poly.degree

Fixes #360. In certain circumstances a `np.int64` was returned from `Poly.degree`. When performing irreducibility checks and computing `q**m`, if either `q` or `m` wasn't an `int` the arithmetic could overflow and the irreducibility check became erroneous.

galois/_polys/_poly.py

@@ -1513,7 +1513,7 @@ def degree(self) -> int:
  if self._nonzero_degrees.size == 0:
  self._degree = 0
  else:
- self._degree = max(self._nonzero_degrees)
+ self._degree = int(max(self._nonzero_degrees))
  elif hasattr(self, "_integer"):
  if self._integer == 0:
  self._degree = 0