Add support for cython, depend on sagemath, instructions for PyPI#19

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mmasdeu wants to merge 15 commits intosagemath:masterfrom

Commits on Jul 20, 2017

Added support for cython, also added dependency on sagemath package.
mmasdeu
committed

Commits on Jul 31, 2017

Finished implementing changes.
mmasdeu
committed

Commits on Aug 3, 2017

Downgraded required Sage version to 7.4
mmasdeu
committed

Commits on Apr 29, 2019

Added line to force use of Python 2.7
mmasdeu
authored

Commits on May 20, 2019

Commits on May 21, 2019

Commits on Feb 19, 2020