mention setuptools_scm bug

Author: flying-sheep

scanpy/_metadata.py

@@ -8,6 +8,7 @@ def refresh_entry_points():
     """\
     Under some circumstances, (e.g. when installing a PEP 517 package via pip),
     pkg_resources.working_set.entries is stale. This tries to fix that.
+    See https://github.com/pypa/setuptools_scm/issues/513
     """
     try:
         import sys