[3.12] gh-117613: Argument Clinic: ensure that defining_class params are positional-only

[neonene]

This is a backport of #117781 and #117950 to 3.12 branch.

3.12 and 3.13 have almost the same issue, but Modules/_sqlite/clinic/connection.c.h has correct NUM_KEYWORDS on 3.12 so far:

/*[clinic input]
_sqlite3.Connection.set_authorizer as pysqlite_connection_set_authorizer

    cls: defining_class
    /
    authorizer_callback as callable: object

Sets authorizer callback.
[clinic start generated code]*/

In old clinic.py, the case above invokes parse_special_symbol(), which overwrites the kind of defining_class from POSITIONAL_OR_KEYWORD to POSITIONAL_ONLY:

cpython/Tools/clinic/clinic.py

Lines 5073 to 5079 in aa26dc3

	# fixup preceding parameters
	for p in self.function.parameters.values():
	if p.is_vararg():
	continue
	if (p.kind != inspect.Parameter.POSITIONAL_OR_KEYWORD and not isinstance(p.converter, self_converter)):
	fail("Function " + self.function.name + " mixes keyword-only and positional-only parameters, which is unsupported.")
	p.kind = inspect.Parameter.POSITIONAL_ONLY

Fixing defining_class params in state_parameter() can cause the parse_special_symbol() above to fail:

Error in file "connection.c" on line 1122:
Function create_function mixes keyword-only and positional-only parameters, which is unsupported.

This PR avoids the error as well.

• Issue: Argument Clinic: Unsafe code generation with defining_class and no slash #117613

[erlend-aasland]

Good sleuthing, @neonene! I applied a style nit. Thank you so much!