Fix warning messages from PyDict_GetItem

[mdickinson]

This PR silences warning messages from PyDict_GetItem in cases where we're trying to look up something that's not hashable.

Note that replacing PyDict_GetItem with PyDict_GetItemWithError means replacing something that can't raise an exception with something that can, but in both cases we already have error paths present that handle the case where an exception is set. (In the validate_trait_map case, raise_traits_error takes care of clearing any existing error. Arguably it should chain the errors instead, but that's a separate discussion.)

Closes #1817

[mdickinson]

It would make sense to review other uses of PyDict_GetItem in ctraits.c. Those where the key being looked up is a string should be fine to leave as-is.

[dpinte]

👍

[mdickinson]

Thanks, @dpinte!