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/.travis.yml b/.travis.yml
index 42d4d16..0fed68b 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,17 +14,19 @@
   include:
     - python: 3.5
       dist: xenial  # Bionic has no Python 3.5
+      script: pip install zipp
 
     # Disabled, see https://github.com/sybrenstuvel/python-rsa/issues/131
     #- python: pypy3.5
     #  dist: xenial  # Bionic has no Python 3.5
 
 install:
-  - pip install poetry
-  - poetry install -v
+  - pip install -U pip setuptools  # https://github.com/pypa/virtualenv/issues/1630
+  - pip install pipenv
+  - pipenv install --dev
 
 script:
-  - poetry run py.test tests/
+  - pipenv run py.test tests/
 
 after_success:
-  - poetry run coveralls
+  - pipenv run coveralls