make PyYAML dependency optional

Author: boegel

setup.py

@@ -107,5 +107,8 @@ def find_rel_test():
     provides=["eb"] + easybuild_packages,
     test_suite="test.framework.suite",
     zip_safe=False,
-    install_requires=["vsc-base >= 2.2.4", "PyYAML >= 3.11"],
+    install_requires=["vsc-base >= 2.2.4"],
+    extras_require = {
+        'yeb': ["PyYAML >= 3.11"],
+    },
 )