Support `setuptools.setup`

[mnonnenmacher]

Currently python-inspector expects that setup.py uses:

from setuptools import setup

setup(
  ...
)

If this syntax is used, python-inspector fails with an exception:

import setuptools

setuptools.setup(
  ...
)

File ".../python_inspector/setup_py_live_eval.py", line 63, in iter_requirements
    assert g["setup"]  # silence warning about unused imports