Remove PySide6 workflow restrictions after next Pyface release

[mdickinson]

After PR #513, the test-with-pypi GitHub Actions workflow has the following restrictions:

• PySide6 < 6.4 is required for Python < 3.11
• PySide6 is not installed at all for Python 3.11

Those restrictions can be removed as soon as Pyface > 7.4.4 is available.

[mdickinson]

It would make sense to solve #555 at the same time as solving this issue. Both issues are waiting only on a Pyface release.