gh-144270: Make SubElement parent and tag positional-only (GH-144845)

The C accelerator implementations use PyArg_ParseTuple, which
inherently enforces positional-only parameters. The Python fallback
allowed these as keyword arguments, creating a behavioral mismatch.

Make the tag parameter of Element.__init__ and the parent and tag
parameters of SubElement positional-only to align with the C
accelerator.