Strings need to be encoded into bytes before the RSA module can operate
on them.

See commit d15a7f3 for the reason why.

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.

This is based on https://github.com/sybrenstuvel/python-rsa/pull/96, with
a few improvements:
  - The minimum of one use of SHA3 in a unit test, to at least touch it at
    some point.
  - Documented the support of SHA3.
  - Only install the third-party library required by Python 3.5 when we're
    running on Python 3.5. Newer Python versions support SHA3 natively.

Poetry generates a setup.py as part of the building process. It's no longer
necessary to keep it in the Git repo.

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