Always return int from Poly.degree

[mhostetter]

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.

Now, #360 runs as follows:

# test_2.py
import galois

f = galois.Poly.Degrees([233, 74, 0])
print(galois.is_irreducible(f))

#initial detection loop
for k in range(1, 233):
    poly = galois.Poly.Degrees([233, k, 0])
    if galois.is_irreducible(poly):
        print("Found one: ", k, poly)

In [1]: %run test_2.py
True
Found one:  74 x^233 + x^74 + 1
Found one:  159 x^233 + x^159 + 1

In [2]: 

[codecov]

Codecov Report

Merging #361 (e9a56d4) into master (c5d930d) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #361   +/-   ##
=======================================
  Coverage   96.76%   96.76%           
=======================================
  Files          41       41           
  Lines        5225     5225           
=======================================
  Hits         5056     5056           
  Misses        169      169           

Impacted Files	Coverage Δ
galois/_polys/_poly.py	95.77% <100.00%> (ø)

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