Pin yapf to 0.20 instead

Author: smurfix

ci/travis.sh

@@ -3,7 +3,7 @@
 set -ex
 
 # See https://github.com/python-trio/trio/issues/334
-YAPF_VERSION=0.17.0
+YAPF_VERSION=0.20.0
 
 if [ "$TRAVIS_OS_NAME" = "osx" ]; then
     curl -Lo macpython.pkg https://www.python.org/ftp/python/${MACPYTHON}/python-${MACPYTHON}-macosx10.6.pkg