FIX: Fix ImportError for gcd

[oyamad]

fractions.gcd has been switched to math.gcd and is now deprecated in python 3.9.

[mmcky]

@oyamad I added python=3.9 to the build matrix for tests but looks like some packages we use aren't release for python=3.9 (via pip)?

Perhaps related to numba/llvmlite#669

[oyamad]

@mmcky Right, numba=0.53 and llvmlite=0.36 have not been released. Their release candidate versions have to be installed manually.

[mmcky]

@mmcky Right, numba=0.53 and llvmlite=0.36 have not been released. Their release candidate versions have to be installed manually.

@oyamad do you think we should wait until we test against python=3.9 to merge?

[oyamad]

@mmcky No, you can merge this PR now (reverting the testing against python=3.9). math.gcd has been there since python=3.5.