[isabelle] Port implmentation to isabelle-client

[wimmers]

There is a new framework for interacting with Isabelle/Server with Python: https://github.com/inpefess/isabelle-client.
It is arguably cleaner than our implementation, so it would be worthwhile to rebase ours on top of the framework.

Are there any caveats?

[wimmers]

The package is on PyPi https://pypi.org/project/isabelle-client/0.1.0/ and thus can easily be installed via pip.