Conda.add may downgrade Python

If a package installed via `Conda.add` is not available for installed Python version, `conda` may downgrade Python:
https://discourse.julialang.org/t/help-understanding-pycall-related-travis-failure/15957

The error is cryptic when it's installed via `pyimport_conda` since the Python installed in disk is different from the one loaded in PyCall.

Maybe we should [pin](https://conda.io/docs/user-guide/tasks/manage-pkgs.html#preventing-packages-from-updating-pinning) Python version and provide a nicer error message pointing to a command that re-builds Conda and PyCall? Or detect change in Python version, automatically run Pkg.build, and then throw an error with message saying that you need to restart Julia?