make mypy invocation py3 compat

Author: mr-c

Makefile

@@ -171,7 +171,7 @@ list-author-emails:
 
 mypy: ${PYSOURCES}
 	rm -Rf typeshed/2.7/ruamel/yaml
-	ln -s $(shell python -c 'import ruamel.yaml; import os.path; print os.path.dirname(ruamel.yaml.__file__)') \
+	ln -s $(shell python -c 'from __future__ import print_function; import ruamel.yaml; import os.path; print(os.path.dirname(ruamel.yaml.__file__))') \
 		typeshed/2.7/ruamel/
 	MYPYPATH=typeshed/2.7 mypy --py2 --disallow-untyped-calls \
 		 --fast-parser --warn-redundant-casts --warn-unused-ignores \

tox.ini

@@ -16,6 +16,7 @@ whitelist_externals = make
 deps =
   mypy-lang>=0.4
   typed-ast
+  -rrequirements.txt
 
 [testenv:py35-lint]
 commands = flake8 schema_salad setup.py