[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.