MNT: Fix issue with mypy 1.18.2

Author: kernc

pdoc/__init__.py

@@ -333,7 +333,7 @@ def get_name(assign_node):
  if not name:
  continue
 
- docstring = inspect.cleandoc(str_node.value.value).strip()
+ docstring = inspect.cleandoc(cast(str, str_node.value.value)).strip()
  if not docstring:
  continue