The third-party library that adds support for this to Python 3.5 is a
binary package, and thus breaks the pure-Python nature of Python-RSA.

This should fix [#147](https://github.com/sybrenstuvel/python-rsa/issues/147).

Poetry breaks no-binary installations of the RSA library, which defeats
the purpose of this library.

See https://github.com/sybrenstuvel/python-rsa/issues/148

Among other changes, this reverts commit
fcf5b7457c70426a242b17db20dd4e34e1055f69.

I also added a workaround for an `ImportError` importing `zipp` on
Python 3.5.

Poetry has a nicer interface, performs more tasks than Pipenv, and is
generally more pleasant to use.

Some of our development dependencies, like Sphinx, have already dropped
support for Python 2. This makes it harder for this project to update
its dependencies. Since Python 2 only has a few more months to live, I
think it's fine to drop support now.

Python 3.4 has already reached its end-of-life date.

Python-RSA now only supports Python 3.5 and newer. Python 3.5 support is
intended to last until its end-of-life date of 2019-09-13:
https://devguide.python.org/#status-of-python-branches

- Added missing test dependencies.
- Updated `.travis.yml` and `tox.ini` to install deps and run with pipenv.
- Added testing on Python 3.7.