functions using context managers wrongly detected as not pure

[martinResearch]

Describe the bug
Variables assigned from contextmanager are not detected as local variables.

To Reproduce
run

from contextlib import contextmanager
from mr_proper.main import check_file


@contextmanager
def foo():
    yield "foobar"


def main():
    with foo() as f:
        return f


if __name__ == "__main__":
    check_file(__file__, recursive=True)

This gives the ouput

foo is pure!
main is not pure because of:
        it uses external name (f)

Expected behavior
main should be detected a pure