False positive: F401 'foo' imported but unused; when marking variable as global variable in a function

False positive: F401 'foo' imported but unused

a.py:
```python
from b import foo

def test():
    global foo
    print(foo)
    foo = 'bar'
    print(foo)

test()
```

b.py:
```python
foo = 'foo'
```

Also resolves [this PyCharm Error](https://youtrack.jetbrains.com/issue/PY-12916)