setup.py: Use setuptools when invoked with bdist_{egg,wheel}.

This makes it easier to build eggs and wheels of 0.19.x releases; the
master branch already uses setuptools.
Merge pull request #265 from msabramo/0.19.x_bdist_setuptools

setup.py: [0.19.x] Use setuptools when invoked with bdist_{egg,wheel}.
fix ref-leak during Py2-style class instantiation
merge 0.19.x branch into master