devel/py-setuptools: Backport fix for namespace_packages

Backport an upstream commit [1] that fixes namespace_packages
for python 3.2+

While I'm here, update TEST_DEPENDS comment and regression-test
invocation.

[1] https://bitbucket.org/pypa/setuptools/pull-request/72

PR: 191637
Submitted by: Melvyn Sopacua <melvyn magemana nl>
Approved by: python@