Improve error message for incompatible TypeVar value

[ilinum]

Fixes #3341

[ddfisher]

@JukkaL what do you think of the updated error message?

[JukkaL]

Looks reasonable! Here's another alternative that would make the error message perhaps even clearer:

Current:     Type argument 1 of "f" (TypeVar "AnyStr") cannot be "object"
Alternative: Value of type variable "AnyStr" of "f" cannot be "object"

It may also make sense to special case typing.AnyStr since it's so common. Here's an idea:

t.py:1: error: Value of type variable "AnyStr" of "f" cannot be "object"
t.py:1: note: Maybe you are mixing "str" and "bytes" values, which is not supported with "AnyStr"?

[ilinum]

Both sounds good!

Regarding the first suggestion though, I think there might be a case where specifying type argument number is useful (when names of two type args are the same).

[ilevkivskyi]

Note that there is PR #3433 that takes a more "global" approach to the AnyStr error message. @quartox are you still working on it?

[quartox]

@ilevkivskyi I will get back to it this weekend.

[ilinum]

I updated the error message as per Jukka's suggestion.

However, I think the additional error message for AnyStr should be handled in #3433 since @quartox is already working on it.

[ddfisher]

LGTM!

[ddfisher]

And agreed -- the AnyStr change can happen in #3433.