Add Python codegen

[pavel-kirienko]

Functionally this is complete and ready for review (sans possible minor CI issues), but the following will need to be completed before this is ready for merging:

• Secure Scott's blessing of the proposed verification suite arrangement. Please consult with verification/python/README.md.
• Fix some minor testing-related shortcomings.
• Add docs.

The codegen templates do not need review as they have been transplanted as-is from PyCyphal. Same goes for the tests themselves.

This changeset also drops support for Python 3.6.

Fixes OpenCyphal/pycyphal#110
Required for OpenCyphal/pycyphal#277
Supersedes #234

[pavel-kirienko]

I would like to pass the value of __version__ defined in nunavut.lang.py.templates to the template context so that I could compare the major version number defined in nunavut_support.py against the version expected by the code generated for a data type. This would be similar to nunavut.version except the version number should be that of the templates. I see that this is supposed to be done via LanguageTemplateNamespace but I don't quite understand the idiomatic way of doing this without altering Nunavut's internals. @thirtytwobits what would be the correct way to go about this?

[thirtytwobits]

I'm not super-thrilled with all the jinja macros (I prefer writing as much code as possible in Python as filters, tests, or other objects) but the results are looking really good.

At some point we'll want to generate python<->c bindings. Do you foresee that being disruptive to the current design of your python?

[pavel-kirienko]

I fixed the ReadTheDocs build issue in the last commit: e6b7c25

The commit adds the RTD config file .readthedocs.yaml specifying a modern Python version. Unfortunately, the change made the ReadTheDocs website unable to find the index.html document in the output root, and indeed it was missing. The least intrusive way to fix that was to lift the index.rst file from docs/ to the root of the repository. This is not pretty and I think a better solution is to actually move all docs-related things down into docs/, including conf.py and requirements.txt, but this is a very invasive change that alters a lot of things (and causes Sphinx to crash with an obscure internal error).

For now, I left it in this state as this is meant to be a quick fix; perhaps we can revisit this later.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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96.6% Coverage
0.0% Duplication