Add 3.7-dev definition

As reported in https://github.com/travis-ci/travis-ci/issues/6599,
the "default" Python branch has moved on to "3.7-dev".

plugins/python-build/bin/python-build

@@ -1655,6 +1655,11 @@ build_package_verify_py36() {
   build_package_verify_py35 "$1" "${2:-3.6}"
 }
 
+# Post-install check for Python 3.7.x
+build_package_verify_py37() {
+  build_package_verify_py36 "$1" "${2:-3.7}"
+}
+
 build_package_ez_setup() {
   local ez_setup="ez_setup.py"
   rm -f "${ez_setup}"