Extract VERSIONS parsing to _utils.py

[srittau]

Preliminary refactoring for fixing #12298.

[AlexWaygood]

Haven't checked line-by-line, but LGTM assuming that you're just moving and deduplicating code!

[srittau]

Haven't checked line-by-line, but LGTM assuming that you're just moving and deduplicating code!

Well, parse_stdlib_versions_file() is basically parse_versions() from mypy_test with hard-coded filename, using pathlib to open and this extra assertion:

assert mod not in result, f"Duplicate module {mod} in VERSIONS"

In check_typecheck_structure, I removed the dupe check, in favor of the assertion above. (Which was already in check_typecheck_structure, so the dupe check was already without function.)