Pre-pin pip/virtualenv to dampen CI jitter

.travis.yml

@@ -15,6 +15,17 @@ install:
 # ~/.local/bin is on $PATH by default, but on OS-X, --user puts it elsewhere
   - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then export PATH=$HOME/Library/Python/2.7/bin:$PATH; fi
   - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then wget https://bootstrap.pypa.io/get-pip.py && sudo python ./get-pip.py; fi
+
+  # Different versions of virtualenv have different seeding algorithms:
+  # https://discuss.python.org/t/-/4146). This got us into trouble when
+  # upgrading from trusty to xenial. I didn't fully unravel the whole thing,
+  # but pinning pip and virtualenv early a) works around the immediate problem,
+  # and b) is probably a good idea, in order to avoid similar problems in the
+  # future.
+
+  - pip install --upgrade pip==19.1.1         # matches tox.ini
+  - pip install --upgrade virtualenv==20.0.25 # pulled out of thin air
+
   - pip list
   - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then pip install --user --upgrade codecov tox setuptools; fi
   - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then pip install --upgrade codecov tox setuptools; fi

tox.ini

@@ -25,7 +25,7 @@ deps =
      # happening at the time.  The versions selected here are just the current
      # versions at the time.  Bumping them to keep up with future releases is
      # fine as long as those releases are known to actually work.
-     pip==19.1.1
+     pip==19.1.1  # keep in sync with .travis.yml
      setuptools==41.0.1
      wheel==0.33.4
      subunitreporter==19.3.2