[setup] Use pyyaml 3.12

Use pyyaml 3.12 for now, since pyyaml 4.1 seems
to contain breaking changes.

Author: mpsonntag

setup.py

@@ -27,7 +27,7 @@
 with open('README.rst') as f:
     description_text = f.read()
 
-install_req = ["lxml", "pyyaml", "rdflib"]
+install_req = ["lxml", "pyyaml==3.12", "rdflib"]
 
 if sys.version_info < (3, 4):
     install_req += ["enum34"]