Try using more recent version of pip, setuptools and virtualenv.
@@ -55,8 +55,9 @@ matrix:
- python3 ./contrib/trigger_rtd_build.py
install:
- - pip install --upgrade "pip==9.0.1"
- - pip install "virtualenv==15.1.0"
+ - pip install --upgrade "pip==19.1.1"
+ - pip install --upgrade "setuptools==41.6.0"
+ - pip install "virtualenv==16.7.7"
- pip install "tox==3.14.0"
- TOX_ENV=py$TRAVIS_PYTHON_VERSION
