I originally picked :pc: as it is short for "python code", but it is
obscure and :py: is not taken, so a much more obvious role can be used
instead. Also, we all typo :pc: as :py: all the time anyway.
This commit also contains a related semantic change: it adds `Shape`
and `ShapeCastable` to the `__all__` list in `amaranth.hdl`. This is
consistent with the policy that is laid out in the new documentation,
which permits such additions without notice.
Co-authored-by: mcclure <mcclure@users.noreply.github.com>
