Removing old, deprecated dependency in conda-build

In PyQt 5.12, the build script depended on a subpackage called pyqt-impl (using pin_subpackage(pyqt-impl, exact=True)), which was renamed to just pyqt in 5.15. So, pyqt 5.12 depends on pyqt-impl 5.12 and pyqt5-sip 4.19, but pyqt 5.15 depends on pyqt5-sip 12.11 and pyqt-impl 15.15 does not exist.

This makes it so that conda (or mamba) cannot update PyQt to the latest version due to version conflicts.

This is a big deal, because matplotlib depends on pyqt, so lots of people would be affected.

How should this issue be fixed?

(A contributor in the issue below mentioned creating an empty package, but that it would only delay the problem.)