[setup] Set pyyaml minimum version

Sucessfully tested pyyaml version 5.1 with python 2.7, 3.7 and 3.8
and use it as the new minimal version of pyyaml.

setup.py

@@ -30,7 +30,7 @@
 with open('README.md') as f:
     description_text = f.read()
 
-install_req = ["lxml", "pyyaml", "rdflib", "docopt", "pathlib"]
+install_req = ["lxml", "pyyaml>=5.1", "rdflib", "docopt", "pathlib"]
 
 if sys.version_info < (3, 4):
     install_req += ["enum34"]