Fix #2998: correctly handle MemberExpr in infer_partial_type

[refi64]

No description provided.

[JukkaL]

Can you look at the build failures?

[refi64]

Looks like it's just testInferListInitializedToEmptyInMethodViaAttribute:

class A:
    def f(self) -> None:
        # Attributes aren't supported right now.
        self.a = [] # E: Need type annotation for variable
        self.a.append(1) # E: Cannot determine type of 'a'
        self.a.append('') # E: Cannot determine type of 'a'

since the last two simply silently exit (which IMO is correct, since cascading errors are annoying). @JukkaL Do you have any objections to me changing it?

[refi64]

I'll take that as a yes...

[gvanrossum]

Thanks again!