travis: Set no-build-isolation pip option

From:
pypa/pip#5229

To avoid failure with Python 3.6:

 Downloading
https://files.pythonhosted.org/packages/c1/36/c31a6ce9411b22a8ac0759c9b5ffda09368d5e0643450bb8ed83c271588c/PyGObject-3.32.1.tar.gz
(698kB) Could not find a version that satisfies the requirement pycairo
(from versions: ) No matching distribution found for pycairo

.travis.yml

@@ -8,6 +8,7 @@ python:
   - "3.7"
 before_install:
   sudo apt-get install -y libgirepository1.0-dev gir1.2-gtk-3.0
+env: PIP_NO_BUILD_ISOLATION=false
 install:
   - python setup.py build
   - pip install tox-travis