Try both 'exec' and 'exec_' methods on QCoreApplication.

[mdickinson]

This PR adds a compatibility fix for Qt6 support. Unfortunately there doesn't seem to be any reasonable way to avoid checking for both exec and exec_ for the near future: PyQt6 only has exec, while PySide2 only has exec_. PySide6 and PyQt5 support both.

No tests because:

• support for Qt6 isn't yet in Pyface master support for Qt6 isn't yet in a Pyface release
• this code is effectively part of the test machinery anyway - it's not currently used by Traits Futures clients

Once Pyface has released support for Qt6, we should add GitHub Actions workflows for testing on the various flavours of Qt versions and wrappers. I'll open a separate issue for that.

[mdickinson]

I'll open a separate issue for that.

#489

[corranwebster]

Looks fine - this is testing code by the look of things, so I assume that performance of testing for exception vs. getattr is not a concern; similarly with not trying to cache the result.