Merge pull request #3 from SekouD/pyup-pin-mypy-0.521

Pin mypy to latest version 0.521

Author: SekouD

requirements_dev.txt

@@ -8,8 +8,9 @@ coverage==4.4.1
 Sphinx==1.6.3
 cryptography==2.0.3
 PyYAML==3.12
+mypy==0.521
 pytest==3.2.2
-mypy
 hypothesis==3.28.1
 beautifulsoup4==4.6.0
 requests
+