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Pants

OK ... so the `Requirement` class actually comes from the pkg_resources module which _should_ be provided by setuptools, and is fwict:
```
$ zipinfo -1 setuptools-33.1.1-py2.py3-none-any.whl | grep pkg_resources
pkg_resources/__init__.py
pkg_resources/_vendor/__init__.py
pkg_resources/_vendor/appdirs.py
pkg_resources/_vendor/pyparsing.py
pkg_resources/_vendor/six.py
pkg_resources/_vendor/packaging/__about__.py
pkg_resources/_vendor/packaging/__init__.py
pkg_resources/_vendor/packaging/_compat.py
pkg_resources/_vendor/packaging/_structures.py
pkg_resources/_vendor/packaging/markers.py
pkg_resources/_vendor/packaging/requirements.py
pkg_resources/_vendor/packaging/specifiers.py
pkg_resources/_vendor/packaging/utils.py
pkg_resources/_vendor/packaging/version.py
pkg_resources/extern/__init__.py
```