Refs #510 - Models can no longer be pickled

[alexhayes]

See #510

Essentially MappingProxyType does not support pickle and thus models could not be pickled anymore.

I've replaced MappingProxyType with dict.

As far as I can tell the access to the only attribute that was using MappingProxyType (schematics.ModelDict._valid) is governed by the API (of course this is Python and we all need to behave as consenting adults) and thus perhaps a read-only dict is not that important?

If it's deemed that this solution is OK I'll fix up the Python 2.6/2.7 failing tests.

[alexhayes]

Just to be sure that MappingProxyType is not supposed to support pickle I've raised this https://bugs.python.org/issue31209

[lkraider]

I am thinking of instead of using different dicts, actually marking the values with metadata to know if they are valid or not. This would simplify the structure back to a single container which could expose iterators for data in each state.

[alexhayes]

@lkraider given what you've said above will that be something that will be happening in the short term or is it more a mid-long term goal?

I'm just wondering if I should expect that this PR will be merged (in which case I might use my fork for the time being knowing that it will be merged) or if I should hold off upgrading from 2.0.0a1 altogether for the time being.

[lkraider]

I want to fix this by removing the MappingProxyType yes, but I would recommend if you require pickling stick with what works for you right now. I'm updating the internals using this PR + tests, so this PR might not merge as-is afterwards.