Support TypeForm

[davidfstr]

The TypeForm PEP draft is nearly done (as of 2025-05-18) and should be implemented soon:

• Alter signatures of public functions to accept TypeForm from typing_extensions or from Python 3.14+

[davidfstr]

Using TypeForm will require users to have a very new typechecker version (as of Dec 2025), so would probably be considered a major/breaking change at this time. After 6 months of stable support of TypeForm in mypy + pyright, I wouldn't consider the change to be major.

In less complex terms: Depending on what calendar date this issue is shipped, it could be considered either a major or a non-major change.