fix: various issues with Python codegen

[lars-reimann]

Closes #386.
Closes #388.

Summary of Changes

• Replace strings in model with richer types
• Overhaul generation of Python code
• Add units tests
• Also include minor changes to the validator, which are not worth putting into a separate PR

Testing instructions

• Test generation of Python code for various inputs

[github-actions]

MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ CREDENTIALS	secretlint	yes		no	3.8s
✅ GIT	git_diff	yes		no	0.01s
✅ KOTLIN	ktlint	25	0	0	62.39s

See errors details in artifact MegaLinter reports on GitHub Action page
Set VALIDATE_ALL_CODEBASE: true in mega-linter.yml to validate all sources, not only the diff

[paul0314]

LGTM!