Allow identical classes to be imported twice

[Michael0x2a]

This pull request fixes #2135.

Previously, if you attempted importing two symbols of the same name twice, mypy would check to see if that symbol was a variable definition or a function and would allow the import if that symbol was previously imported.

This commit extends this check slightly by also allowing identical classes to be imported twice.

[gvanrossum]

Interesting. I'll merge this in a sec, but I am not really comfortable with the (existing!) behavior that overlapping imports are allowed when they import different things that happen to have the same type. The exception is meant for conditional imports, e.g.

if ...:
    from x import a
else:
    from y import a

but it seems it also allows plain overrides:

from x import a
from y import a

where x.a and y.a are unrelated but happen to have the same type.

But that's not this PR's fault, and it looks like colliding classes are always considered to be different, so we're actually better off for classes than for functions!