[Python Backend] Port QRegExp to QRegularExpression

PythonHighlighter::highlightBlock(): Make the regex's static const to
improve performance, assuming this is a high traffic code path. Also use
QVector instead of QList.

All unit tests pass except for testlua (fails the same way on master);
and testworksheet which hangs (on master too).

A quick test with a pythong session in cantor shows that syntax highlighting
still works.