Add python version to README

[Mar4eli]

Hello, thank you for your lib!
Can you please add required python version for your readme. I have had a hard time with python 3.5 and syntax error in lib, python 2.7 (via anaconda) works perfectly.

[augustt198]

I've only tested the project using python 2.7. That said, I don't think it would be too hard to run it in python 3.5. I think the only thing that would have to be updated is the antlr4 dependency (using antlr4-python3-runtime instead of antlr4-python2-runtime)

[LennyLip]

I've been installed latex2sympy with python 3 and have only 83/162 STRINGS tests PASSED .
Can anybody else confirm it?