ez_setup.use_setuptools() fails if it needs to download a newer setuptools

Originally reported by: agronholm (Bitbucket: agronholm, GitHub: agronholm)

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In setup.py:

#!python

import ez_setup
ez_setup.use_setuptools('20.3')

Then, with setuptools 20.1.1 installed:

$ python setup.py
Traceback (most recent call last):
  File "setup.py", line 38, in <module>
    ez_setup.use_setuptools('20.3')
  File "/home/alex/checkout/crossbar/ez_setup.py", line 173, in use_setuptools
    return _do_download(version, download_base, to_dir, download_delay)
  File "/home/alex/checkout/crossbar/ez_setup.py", line 132, in _do_download
    import setuptools
  File "build/bdist.linux-x86_64/egg/setuptools/__init__.py", line 11, in <module>
  File "build/bdist.linux-x86_64/egg/setuptools/extern/__init__.py", line 1, in <module>
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 48, in <module>
  File "/home/alex/virtualenv/crossbar/local/lib/python2.7/site-packages/pkg_resources/extern/__init__.py", line 43, in load_module
    mod = sys.modules[extant]
AttributeError: 'NoneType' object has no attribute 'modules'

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• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/519

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Merged in agronholm/setuptools/agronholm/remove-the-import-hook-when-unloading-th-1458177009332 (pull request #182)
. Fixes #519.
Remove the import hook when unloading the old pkg_resources modules

Original comment by jaraco (Bitbucket: jaraco, GitHub: jaraco):

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Merged in agronholm/setuptools/agronholm/remove-the-import-hook-when-unloading-th-1458177009332 (pull request #182)
. Fixes #519.
Remove the import hook when unloading the old pkg_resources modules