.travis.yml: Try using pip instead of pipenv

The Pipfile knows it was created with Python 3.8, yet we're running
with multiple Python versions on Travis. I'm curious if would work
better to use pip to install dependencies instead of pipenv in this
case.

.travis.yml

@@ -5,8 +5,7 @@ python:
   - "3.7"
   - "3.8"
 install:
-  - "pip install pipenv --upgrade-strategy=only-if-needed"
-  - "pipenv install --dev"
+  - "pip install -r requirements-dev.txt"
 script: pytest
 
 # vim: ts=2 sw=2 et