sys.meta_path is not cleaned up after install and before test

Originally reported by: icordasc (Bitbucket: icordasc, GitHub: Unknown)

Recently, requests added some logic to it's vendored module which adds a class to sys.meta_path.

Current versions of requests distribute a setup.py file which imports requests to retrieve the version. As such, sys.meta_path will have the instance of VendorAlias still when the tests are run. When requests is next imported, it will fail (because sys is None and the code in load_module (from the old instance of VendorAlias) is expecting it to not be None. If we re-import sys other module globals no longer exist).

In short, after the initial step (where dependencies are downloaded and placed in .eggs) sys.meta_path should be cleaned up by setuptools to prevent issues like this.

Discussion of the above bug is mostly on this pull request but also on this issue.

/cc @dstufft

• Bitbucket: https://bitbucket.org/pypa/setuptools/issue/363

Start by tracing the dependency-install step that places packages in .eggs and the point before tests run, then inspect how sys.meta_path is handled. Use requests/setup.py and requests/packages/init.py as the reproduction context. Done means the added VendorAlias state is cleaned up before tests so a later requests import does not fail.