[PyQt] ANN: PyQt v5.12.3 Released

[Phil Thompson]

PyQt v5.12.3 has been released. This is a minor functional and bug-fix 
release.

- Added support for Qt v5.12.4.

- Added support for Python v3.8.

- QCoreApplication (and sub-classes) can be used as a context manager.

- Added the missing WA_ContentsMarginsRespectsSafeArea enum member.

There are no corresponding releases of the other PyQt packages but their 
wheel have been update to Qt v5.12.4. To update to the latest versions 
use pip's '--force-reinstall' option.

Phil