Add prominent warnings about having theano/pymc3 installed in parallel

[michaelosthege]

This is similar to 33c0e35.

Example output:

(pm3v4) E:\Repos\pm3v4>python -c "import pymc"
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Your Python environment has Theano(-PyMC) 1.1.2 installed, but you are importing PyMC 4.0.0 which uses Aesara as its backend.
For PyMC 4.0.0 to work as expected you should uninstall Theano(-PyMC).
See https://github.com/pymc-devs/pymc3/wiki for installation instructions.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
You are importing PyMC 4.0.0, but your environment also has the legacy version PyMC3 3.11.4 installed.
For PyMC 4.0.0 to work as expected you should uninstall PyMC3.
See https://github.com/pymc-devs/pymc3/wiki for installation instructions.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
You are running the v4 development version of PyMC which currently still lacks key features. You probably want to use the stable v3 instead which you can either install via conda or find on the v3 GitHub branch: https://github.com/pymc-devs/pymc/tree/v3