Make types for generic View mixins less strict

[mkurnikov]

Fixes #152

@treyhunner, I believe this PR fixes 2 out of 3 errors presented in the issue. The last one, with the case of

class Mixin1:
    model: Type[Model]
class Mixin2:
    model: MyModel
class MyView(Mixin1, Mixin2):
    pass

is an upstream issue. See, for example, this one python/mypy#1237. The only thing we can do here is to make model an Any, which trading false-positive for false-negative, I don't know...