Upgrade PyYAML to 6.0.1

This was causing install issues with newer versions of Python /
setuptools; see https://github.com/yaml/pyyaml/issues/723.

Thanks to Rui Chen <rui@chenrui.dev> for reporting this issue in
https://github.com/anishathalye/dotbot/pull/340.

lib/pyyaml

@@ -1 +1 @@
-Subproject commit 2f463cf5b0e98a52bc20e348d1e69761bf263b86
+Subproject commit c42fa3bff1eabdb64763bb1526d9ea1ccb708479

setup.py

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