check-python-files misses subdirectories of mbedtls_framework

[gilles-peskine-arm]

Since #25 there is an extra directory containing Python modules: framework/scripts/mbedtls_framework/code_wrapper. The modules in that directory are checked with mypy because it checks generate_psa_wrappers.py which imports all the modules in the code_wrapper subdirectory. But the modules under code_wrapper are not checked by pylint.

The goal of this issue is:

• Change check-python-files.sh to cover all the existing Python files in a way that is future-proof if we add more subdirectories to mbedtls_framework
• Fix the issues reported by Pylint.

[gilles-peskine-arm]

I've gotten started on this, and in fact I'm mostly done, with one exception which I'm still working on: this is exposing that the multiple inheritance in PSALoggingWrapper and PSALoggingTestWrapper isn't done right.

[gilles-peskine-arm]

Resolved by #283: d393ebd and c77ecf0 to fix the problems in the code_wrapper subdirectory, and 345b13a to check mbedtls_framework recursively.