Upgrade PyYAML to 6.0.1

This was causing install issues with newer versions of Python /
setuptools; see yaml/pyyaml#723.

Thanks to Rui Chen <[email protected]> for reporting this issue in
#340.

Author: anishathalye

lib/pyyaml

@@ -52,7 +52,7 @@ def find_version(*file_paths):
         "wheel>=0.31.0",
     ],
     install_requires=[
-        "PyYAML>=5.3,<6",
+        "PyYAML>=6.0.1,<7",
     ],
     extras_require={
         "dev": {