Python version support policy

[Aaron Meurer]

I've written up our policies for supporting major Python versions here
https://github.com/sympy/sympy/wiki/Python-version-support-policy.
Everything there has been our policy for a while, but it was never
written down.

The policy is basically to drop support for major Python versions when
they have reached end-of-life. For Python 2, this means that the last
version of SymPy released in 2019 will be the last version to support
Python 2.

Let me know if there are any questions or concerns.

Aaron Meurer