Allow --additional-flags to vary between old & new versions of typechecker

[davidfstr]

I'd like to run mypy_primer such that it calls the old version of mypy with a different set of flags than the new version of mypy.

For example, when working on PEP 747 (TypeForm) it would be useful to run the old mypy with --additional-flags="X" and the new mypy with --additional-flags="X --enable-incomplete-feature=TypeForm" so that I can see if enabling my incomplete feature in the new version of mypy causes problems in the projects that mypy_primer knows about. I can't enable my incomplete feature in the old version of mypy because it doesn't know about the feature.

Proposal:

• Extend mypy_primer's CLI to support options --old-additional-flags="X" and --new-additional-flags="Y" to allow different flags to be passed to the old & new versions of mypy.
• Maintain in mypy_primer's CLI the option --additional-flags="Z", which is treated as equivalent to --old-additional-flags="Z" --new-additional-flags="Z"

[hauntsaninja]

Looks like you're doing this in mypy CI, in which case maybe the easier solution is to just enable the flag by default. (Either way you'll have to push a change to the PR removing once you're happy with the primer diff)

[davidfstr]

maybe the easier solution is to just enable the flag by default

Agreed. I'll do that.