Switch from Pipenv to Poetry

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

Author: sybrenstuvel

.travis.yml

@@ -19,11 +19,11 @@ matrix:
     #  dist: xenial  # Bionic has no Python 3.5
 
 install:
-  - pip install pipenv
-  - pipenv install --dev
+  - pip install poetry
+  - poetry install -v
 
 script:
-  - pipenv run py.test
+  - poetry run py.test tests/
 
 after_success:
-  - pipenv run coveralls
+  - poetry run coveralls