chore(py_pytest_main): Refactor to allow for linting

[arrdem]

The pytest driver template isn't valid Python, which makes it unlintable, which is part of why it has unmitigated typing issues which become visible when users adopt rules_mypy and equivalents.

Rework the driver template so that in its pre-expansion form it's valid Python and impose linting on it.

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Changes are visible to end-users: no

Test plan

TBD