Drop pypy3 (temporarily)

[Nurdok]

PyPy3 currently only implements the equivalent of CPython 3.2, which we don't support. Pip has also dropped support for PyPy3, resulting in the following build error:

/home/travis/virtualenv/pypy3-2.4.0/site-packages/virtualenv_support/pip-9.0.1-py2.py3-none-any.whl/pip/_vendor/pkg_resources/__init__.py:83: UserWarning: Support for Python 3.0-3.2 has been dropped. Future versions will fail here.
pip requires Python '>=2.6,!=3.0.*,!=3.1.*,!=3.2.*' but the running Python is 3.2.5

Support for PyPy3 will be restored when it is supported by Pip, and when it will implement a newer equivalent version for CPython (3.3 or higher).

[peterjc]

I noticed this issue noted in the v2.0.0 release notes - but PyPy3.5 v5.7 is out now and supports the equivalent of CPython 3.5

This isn't available on TravisCI by default yet, but that can be worked around - see travis-ci/travis-ci#6727