Use the select interface when using entry points

[rahulporuri]

This was missed in #991 and is needed to fully resolve #952 . The change introduced in this PR removes the deprecation warning raised by importlib_metadata from the test run as well.

2021-07-16T05:11:29.3287021Z test_core_plugins (pyface.tests.test_toolkit.TestToolkit) ... /home/runner/work/pyface/pyface/.edm/envs/pyface-test-3.6-pyqt5/lib/python3.6/site-packages/pyface/tests/test_toolkit.py:36: DeprecationWarning: SelectableGroups dict interface is deprecated. Use select.
2021-07-16T05:11:29.3290051Z   plugins = {ep.name for ep in entry_points()['pyface.toolkits']}
2021-07-16T05:11:29.3290882Z ok

[corranwebster]

LGTM

[rahulporuri]

thanks for the review @corranwebster