Add separate requirements file for Python 3

Separate requirements-py3.txt file allows to specify different
dependency list for python 3. For example, now python-memcached doesn't
support py3k and some other dependencies may appear later.  Also tox.ini
was changed to use proper requirements file for py33 testenv.

Partially-implements: blueprint py3k-support
Change-Id: I55aa0e3364124f27532465fcaa0b9316c7dc67e6
Merge "Add separate requirements file for Python 3"