multiple discarded variables with undescore (_) cause Incompatible types in assignment

[asottile]

Feature

Here is a silly example:

x = ((1, 'a', 1.4),)

for _, _, y in x:
    print(y)

The current output (mypy 0.800)

$ mypy t.py 
t.py:3: error: Incompatible types in assignment (expression has type "str", variable has type "int")
Found 1 error in 1 file (checked 1 source file)

Here's a real world example:

import pkgutil

from somemod import _plugins

# trigger an import of all of the plugins
# https://github.com/python/mypy/issues/1422
plugins_path: str = _plugins.__path__  # type: ignore
mod_infos = pkgutil.walk_packages(plugins_path, f'{_plugins.__name__}.')
for _, name, _ in mod_infos:
    __import__(name, fromlist=['_trash'])

pyupgrade/_data.py:101: error: Incompatible types in assignment (expression has type "bool", variable has type "Union[MetaPathFinder, PathEntryFinder]")

Pitch

The discarded _ variable here is immediately clobbered before it could possibly be read, I'd like this to pass without error

(I couldn't find this searching, though I suspect there's a duplicate -- sorry in advance).

[hauntsaninja]

We already support some forms of this, so seems reasonable to get it to work here as well (#465). As a workaround, you can put _: Any on the line before

[asottile]

ah, I see from that issue that another workaround is to sequester the loop into a function instead of executing it at the module scope