unpin autodoc-traits

we don't pin any other doc dependencies,
and the bug preventing build has already been fixed

Author: minrk

doc/doc-requirements.txt

@@ -9,7 +9,7 @@
 # environment, which it is on ReadTheDocs.
 
 # Documentation specific packages
-https://github.com/jupyterhub/autodoc-traits/archive/75885ee24636efbfebfceed1043459715049cd84.zip
+https://github.com/jupyterhub/autodoc-traits/archive/HEAD.zip
 myst-parser
 pydata-sphinx-theme
 sphinx-copybutton