Moving back to Pipenv to manage dependencies

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

See #148

.travis.yml

@@ -20,11 +20,11 @@ matrix:
     #  dist: xenial  # Bionic has no Python 3.5
 
 install:
-  - pip install poetry
-  - poetry install -v
+  - 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

Pipfile

@@ -0,0 +1,21 @@
+[[source]]
+url = "https://pypi.org/simple"
+verify_ssl = true
+name = "pypi"
+
+[packages]
+"pyasn1" = ">=0.1.3"
+"pysha3" = {version = "~=1.0, >=1.0",markers = "python_version < '3.6'"}
+
+[dev-packages]
+coveralls = "~=1.8, >=1.8"
+"Sphinx" = "~=2.1, >=2.1"
+"pathlib2" = {version = "~=2.3, >=2.3.4",markers = "python_version < '3.6'"}
+"pytest" = "~=5.0, >=5.0"
+"pytest-cov" = "~=2.7, >=2.7"
+"tox" = "~=3.13, >=3.13"
+"mypy" = "~=0.720, >=0.720"
+"flake8" = "~=3.7, >=3.7"
+
+[requires]
+python_version = "3.7"