Toolkit selection is broken on Python 3.9

[mdickinson]

Since #991, toolkit selection is broken on Python 3.9. Running the test suite shows:

(pyface) mdickinson@mirzakhani pyface % python -V
Python 3.9.6
(pyface) mdickinson@mirzakhani pyface % python -m unittest
E
======================================================================
ERROR: pyface (unittest.loader._FailedTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/mdickinson/Enthought/ETS/pyface/pyface/__init__.py", line 61, in load_tests
    from pyface.toolkit import toolkit_object  # noqa: F401
  File "/Users/mdickinson/Enthought/ETS/pyface/pyface/toolkit.py", line 23, in <module>
    toolkit = toolkit_object = find_toolkit("pyface.toolkits")
  File "/Users/mdickinson/Enthought/ETS/pyface/pyface/base_toolkit.py", line 259, in find_toolkit
    plugin for plugin in importlib_metadata.entry_points().select(group=entry_point)
AttributeError: 'dict' object has no attribute 'select'

----------------------------------------------------------------------
Ran 1 test in 0.000s

FAILED (errors=1)

The issue is that in Python 3.9, importlib.metadata doesn't provide the select interface (it's new in Python 3.10).

I think in the short term, #991 should be reverted while we figure out the right fix. That may be to ignore the deprecation warnings, or we may want to deliberately continue to use importlib_metadata instead of importlib.metadata on Python 3.9. (It's annoying to have to do so, though - we'd really like to be able to drop use of importlib_metadata sooner rather than later.)

Related: python/importlib_metadata#298

[mdickinson]

The right fix may be something similar to this: astropy/astropy#11534