[WIP] Avoid using __dict__

[thedrow]

__dict__ is an implementation detail. We should avoid using it.

Fixes #93.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage remained the same at 97.689% when pulling c8b0476 on thedrow:patch-1 into e3814d2 on uber:master.

[coveralls]

Coverage remained the same at 97.689% when pulling c8b0476 on thedrow:patch-1 into e3814d2 on uber:master.

[thedrow]

Before merging this I need to add tests.

[thedrow]

So it turns out this library indeed doesn't support partial doubles.
We can fix the tests because now we get a proper error.

I'd appreciate a code review before going any further.

[toddsifleet]

@thedrow what do you mean by "indeed doesn't support partial doubles"? Do you have an example?

[toddsifleet]

I'm not sure this will solve the problem, because we get errors like
AttributeError: 'UserWithSlots' object attribute 'arbitrary_callable' is read-only

I have to think more about how to support objects with __slots__

[thedrow]

Exactly my point but at least with this PR we get a clearer error message.