Readable and writable binary files cause false positives

[JukkaL]

This generates an error when using master, even though it doesn't generate error with mypy 0.931:

with open('x', 'rb') as f:
    s = f.read()
with open('y', 'wb') as f:  # Incompatible types in assignment
                            # (expression has type "BufferedWriter",
                            # variable has type "BufferedReader")
    f.write(s)

This is because of a typeshed/plugin change. The new types are better than what we had before, so any improvement should be in mypy itself. This seems worth fixing at least if the fix isn't too complex, since code like the above example is fairly common.

My first idea would be to create a separate scope for each with statement, so that the types of the two variables named f could be inferred independently. We could internally rename the different variables to make them distinct, similar to what we do when using --allow-redefinition.

If the variable is used outside the with statement, we may want to revert to the current behavior for backward compatibility, since renaming in this case would require much more complex logic. This is a bit ad hoc, but the case should be quite rare.