Make python style compliant

Brings the python files in this repo into compliance with the
checks in `sel4_tools`.

Convert to SPDX license tags

This includes marking all documentation files CC-BY-SA-4.0.

Fix licenses

Removed broken __getitem__ methods

The Spec and ObjectAllocator classes defined __getitem__ methods which
were never used and ended up invoking __getitem__ on a set, which is not
defined.

python-capdl-tool: update Spec.py to use new arch abstraction

python-capdl: Normalise the architecture being assigned to a spec.

This commit allows callers to be a bit fuzzy when specifying the architecture
of a CapDL specification. It is intended to support upcoming changes to
camkes-tool to unify support for ARM's Hyp mode.

Related to JIRA CAMKES-394, CAMKES-334, SELFOUR-297

python-capdl: Enable Python 3 compatibility settings from `__future__`.

This change enables a variety of Python 3 compatibility settings from
`__future__` when running Python 2. Essentially this forces all existing code
under Python 2 to conform to the Python 3 semantics. This makes bilingual
compatibility easier.

python-capdl: Make all intra-package imports relative.

This progresses compatibility with Python 3 to the point where this package can
be imported. I haven't explored further, but I believe the package still cannot
be used correctly under Python 3. More changes to come.

Moved python-capdl-tool into subdirectory