Regression since v0.520 due to PR #3700: Name 'blah' not defined

Example:
```py
class A(object):
    def f(self):
        self.attr = 1
    attr = 0  # error: Name 'attr' is not defined

class B(object):
    attr = 0  # no error
    def f(self):
        self.attr = 1
```
This has no errors in v0.520 but has an error at the indicated line at f127e2cac5277c1d008da50611532167891cae48, i.e. it is caused by #3700.