Solve the Travis failure with 3.7 from deadsnakes

.travis.yml

@@ -1,7 +1,12 @@
-sudo: false
+sudo: required
+dist: xenial
 language: python
 cache: pip
-before_script:
+before_install:
+- if [[ $TRAVIS_PYTHON_VERSION == '3.7-dev' ]]; then sudo add-apt-repository ppa:deadsnakes/ppa -y; fi
+- if [[ $TRAVIS_PYTHON_VERSION == '3.7-dev' ]]; then sudo sudo apt-get update; fi
+install:
+- if [[ $TRAVIS_PYTHON_VERSION == '3.7-dev' ]]; then source .travis_py37_workaround.sh; fi
 - pip install coverage coveralls flake8 flake8-bugbear mypy
 - pip install -e .
 script:

.travis_py37_workaround.sh

@@ -0,0 +1,12 @@
+echo "The ready-made virtualenv is not the one we want. Deactivating..."
+deactivate
+
+echo "Installing 3.7 from deadsnakes..."
+sudo apt-get --yes install python3.7
+
+echo "Creating a fresh virtualenv. We can't use `ensurepip` because Debian."
+python3.7 -m venv ~/virtualenv/python3.7-deadsnakes --without-pip
+source ~/virtualenv/python3.7-deadsnakes/bin/activate
+
+echo "We ensure our own pip."
+curl -sSL https://bootstrap.pypa.io/get-pip.py | python3.7