devel/py-sip: doesn't honor sip_dir

pyqt.mk had a long standing bug, where the sip-dir was not properly set
for the port devel/py-sip. The underlying issue was the following:

When using pyqt.mk consumers would specify

USES=pyqt:<version>

To also have easy access for the data from pyqt.mk in the py-sip port, we allowed
version=sip, which only unhides a very limited scope of pyqt.mk. However, we also
set

_VERSION_SUBDIR_REL= PyQt${_PYQT_VERSION}/${PYTHON_VER}

and used that in

_SIPDIR_REL= share/${_VERSION_SUBDIR_REL}/sip
PYQT_SIPDIR= ${PREFIX}/${_SIPDIR_REL}

Therefore, py-sip got a very mangled PYQT_SIPDIR value passed to its configure
script.

Big thanks to Loïc for figuring out the issue :)

PR: 223409
Submitted by: Loïc Bartoletti <lbartoletti@tuxfamily.org>