qt5

Created originally on Bitbucket by kiilerix (Mads Kiilerich)

The last Qt4 release was in 2011, and Qt5 was released in 2012.
Qt4 support is bitrotting and will eventually go away. Fedora 32 no longer has PyQt4.Qsci, so in order to keep hgview packaged, we have to upgrade.
Qt5 is so old and mature that a full switch to Qt5 should be there - there shouldn't be any reason to support 4 and 5 side by side.
But also note that PyQt5 version 5.14.0 requires Python >=3.5, so an upgrade to latest PyQt5 will effectively drop Python 2 support. That seems very reasonable these days.

This PR migrates the Qt4 support to Qt5. Everything works in my (limited) testing.

Perhaps first apply the first curses/py3/qt4 changes, test it, and make a final 1.13.2 release. (I no longer have py2 / qt4 readily available and haven't tested.) (The first 3 qt5-ish changes are refactorings that also should work with py2/qt4 - that would be nice to get tested too.)

If the other changes works for you in review and testing and land, we can perhaps follow up with some more py2 cleanup while waiting for further feedback, and then get 1.14.0 out in a week?