Move most of min_requirements.py to the framework

[mpg]

We want to keep scripts/min_requirements.py in each branch (3.6 and development), as that's the entry point that the CI is calling. But we want to move most of its code to the framework, as a module that can be imported. So, the end result would probably have scripts/min_requirements.py in each branch looking like

import framework_scripts_path # pylint: disable=unused-import
from mbedtls_framework import min_requirements

min_requirements.main()

and then the current body of scripts/min_requirements.py would be moved to the framework repo, in scripts/mbedtls_framework (as a non-executable file).

In terms of history, we probably want to first completely move min_requirements.py to the framework (so that its full history is present there) and then add back the new stub in 3.6 and development.