You should check out SageMath.
It's a math software (very much like Mathematica and in some ways better), which is open source and built on python. There's a freemium cloud service for it too called Cocalc!
(if you don't want to run up your own instance, the cloud service is really nice!)
You beat me to it! But Sage also has a win version, although online comments suggest it is very slow on windows.
There is a wikipedia list of similar software:
https://en.wikipedia.org/wiki/List_of_computer_algebra_systems
https://en.wikipedia.org/wiki/Mathematical_software
and
https://en.wikipedia.org/wiki/List_of_open-source_software_for_mathematics
Thank you, guys, will look into it!