Stop using deprecated trait handlers

[jwiggins]

Closes #433.

There's still a deprecation warning about using TraitMap, but this is coming from traits internal code, so I'm not sure anything can be done within enable.

/Users/jwiggins/.edm/envs/enable-test-3.6-pyqt5/lib/python3.6/site-packages/traits/traits.py:330: DeprecationWarning: 'TraitMap' trait handler has been deprecated. Use Map instead.
  other.append(TraitMap(map))

[jwiggins]

Failure seems unrelated to the changes:

Download action repository 'actions/checkout@v2'
Error: An action could not be found at the URI 'https://api.github.com/repos/actions/checkout/zipball/5a4ac9002d0be2fb38bd78e4b4dbde5606d7042f'

[rahulporuri]

@jwiggins i'm going to rerun the jobs to see if the problem fixes itself - but it'll be interesting if this happens again - Update : Looks like that fixed the CI failure.

[rahulporuri]

LGTM

[jwiggins]

Thanks for the review

[kitchoi]

There's still a deprecation warning about using TraitMap, but this is coming from traits internal code, so I'm not sure anything can be done within enable.

The deprecation warning coming from Traits is reverted as part of Traits 6.2. See enthought/traits#1365

The move from TraitPrefixMap to PrefixMap may not be possible in cases where callables are also used, see enthought/traits#1336
However I think that edge case does not apply here, so the switch here is fine.

I am sorry I should have updated #433 on that particular warning.

[jwiggins]

No worries @kitchoi. Thanks for adding an explanation here.