pipeline / pipenv: explicitly set python version to keep pipenv from picking the highest available

https://github.com/WanzenBug/pylint-import-requirements/commit/b7fe5320b0538cff1b7f0c4963cb3d4b68c9a978#diff-354f30a63fb0907d4ad57269548329e3

.travis.yml

@@ -31,7 +31,7 @@ matrix:
 
 install:
 - pip install pipenv==2020.6.2
-- pipenv sync --dev
+- pipenv install --python "$TRAVIS_PYTHON_VERSION" --deploy --dev
 - if [ ! -z "$PANDAS_VERSION" ]; then
     pipenv install --selective-upgrade "pandas==$PANDAS_VERSION";
   fi