Moving back to Pipenv to manage dependencies

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.
diff --git a/setup.py b/setup.py
new file mode 100755
index 0000000..7a6b872
--- /dev/null
+++ b/setup.py
@@ -0,0 +1,67 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+#  Copyright 2011 Sybren A. Stüvel <sybren@stuvel.eu>
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      https://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+# io.open is needed for projects that support Python 2.7. It ensures open()
+# defaults to text mode with universal newlines, and accepts an argument to
+# specify the text encoding Python 3 only projects can skip this import.
+from io import open
+from setuptools import setup
+
+with open('README.md', encoding='utf-8') as f:
+    long_description = f.read()
+
+if __name__ == '__main__':
+    setup(name='rsa',
+          version='4.2-dev0',
+          description='Pure-Python RSA implementation',
+          long_description=long_description,
+          long_description_content_type='text/markdown',
+          author='Sybren A. Stuvel',
+          author_email='sybren@stuvel.eu',
+          maintainer='Sybren A. Stuvel',
+          maintainer_email='sybren@stuvel.eu',
+          url='https://stuvel.eu/rsa',
+          packages=['rsa'],
+          license='ASL 2',
+          classifiers=[
+              'Development Status :: 5 - Production/Stable',
+              'Intended Audience :: Developers',
+              'Intended Audience :: Education',
+              'Intended Audience :: Information Technology',
+              'License :: OSI Approved :: Apache Software License',
+              'Operating System :: OS Independent',
+              'Programming Language :: Python',
+              'Programming Language :: Python :: 3',
+              'Programming Language :: Python :: 3.5',
+              'Programming Language :: Python :: 3.6',
+              'Programming Language :: Python :: 3.7',
+              'Programming Language :: Python :: Implementation :: CPython',
+              'Programming Language :: Python :: Implementation :: PyPy',
+              'Topic :: Security :: Cryptography',
+          ],
+          install_requires=[
+              'pyasn1 >= 0.1.3',
+          ],
+          entry_points={'console_scripts': [
+              'pyrsa-priv2pub = rsa.util:private_to_public',
+              'pyrsa-keygen = rsa.cli:keygen',
+              'pyrsa-encrypt = rsa.cli:encrypt',
+              'pyrsa-decrypt = rsa.cli:decrypt',
+              'pyrsa-sign = rsa.cli:sign',
+              'pyrsa-verify = rsa.cli:verify',
+          ]},
+
+          )