gen_isabelle_root: add option for session dirs

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

regression: force use of python3

Python 2 has passed its sunset date, and many distributions are
withdrawing support for Python 2.

PEP 394 recommends distributions always install versioned interpreter
commands (e.g. `python3`), but does not make a recommendation about
whether or not an unversioned command (`python`) should exist, or what
version it should run.

It therefore seems advisable to explicitly run scripts using the
`python3` command, for scripts that are compatible with Python 3.

Here, we do this for Python scripts used by `run_tests`. For this to
work, some scripts have been updated in ways that will break Python 2
compatibility. But for some other scripts which were already compatible
with both Python 2 and 3, we have not yet removed Python 2
compatibility. There are also miscellaneous scripts that are not used by
`run_tests`, and these have not yet been updated to Python 3.

Signed-off-by: Matthew Brecknell <Matthew.Brecknell@data61.csiro.au>

style: pep8 style for python files

licenses: convert license tags to SPDX

Isabelle2018: CParser

Isabelle2017: add session-qualified imports to gen_isabelle_root

Import release snapshot.