Fix broken PyVista import

[banesullivan]

Description

Fixes broken PyVista/PyVistaQt import with latest releases.

Checklist

• My code follows the PEP 8 style guidelines.
• My code uses type hinting for function and method arguments and return values.
• My code contains descriptive and helpful docstrings
  which are formatted per the Google Python Style Guidelines.
• I have created tests which entirely cover my code.
• The test code either 1. demonstrates at least one valuable use case (e.g. integration tests)
  or 2. verifies that outputs are as expected for given inputs (e.g. unit tests).
• New and existing tests pass locally with my changes.

[banesullivan]

I did not actually test this... Just my thought on how to fix it since this code moved recently in PyVista and is no longer located in the pyvista.plotting.theme submodule. It should be accessible from pyvista.plotting directly across versions though

Pretty sure the breaking change happened in pyvista/pyvista#1310

[banesullivan]

PyVistaQt switched to using QtPy to allow any Qt backend. I added pyqt5 as a dependency here so that QtPy has Qt bindings to leverage.

[banesullivan]

Feel free to use a different PyQt backend per https://github.com/spyder-ide/qtpy#requirements