Allow `FrameType` to be instantiated

Feature or enhancement

Proposal:

For some reason, FrameType is not allowed to be instantiated. This creates problems when trying to (for example) control an exception traceback, which is common for debuggers, among other uses. This is also inconsistant with most other internal types that exposed to Python and in the types module.

The only reason I can think of is because PyFrame_New requires a PyThreadState object, which is not exposed to Python. However, there are several ways around this:

• Run PyThreadState_GetUnchecked, raising RuntimeError (or another error) if NULL is returned
• Expose frames' PyThreadState to the interpreter
• Pass another frame as a parameter, and get its thread state

This should be relatively trivial to implement and I can't think of any downsides, backwards compatibility concerns, or maintenance overhead. I can probably write a PR if one of the solutions to the thread state parameter is decided.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Start by reviewing the FrameType instantiation path and the PyFrame_New and PyThreadState_GetUnchecked entry points named in the issue. Decide how instantiation should obtain or validate a thread state, then confirm that FrameType can be created from Python without breaking the stated traceback and debugger use cases.