PyYAML works on 3.7 now. Install patched defusedxml for 3.8-dev

.travis.yml

@@ -16,10 +16,12 @@ before_install:
   -in settings.yml.enc -out settings.yml -d
 
 install:
-# Sometimes, only lxml master branch compiles with Python nightly
+# Only lxml master branch compiles with Python nightly
 # - if [[ "$( python --version | grep 'a' )" ]] ; then pip install cython && pip install git+https://github.com/lxml/lxml.git ; fi
 # Only PyYAML master branch compiles with Python nightly
-- if [[ "$( python --version | grep 'a' )" ]] ; then pip install git+https://github.com/yaml/pyyaml.git ; fi
+# - if [[ "$( python --version | grep 'a' )" ]] ; then pip install git+https://github.com/yaml/pyyaml.git ; fi
+# Fix for changed XMLParser signature on Python nightly
+- if [[ "$( python --version | grep 'a' )" ]] ; then pip install git+https://github.com/ecederstrand/defusedxml.git@patch-2 ; fi
 - pip install .
 # Install test dependencies manually since we're calling tests/__init__.py directly in the 'script' section
 - pip install PyYAML requests_mock psutil