Fix type expected by stubgenc (#8888)

In investigating a crash when running on a PyBind11 module, I noticed 
a mismatch in the types; infer_prop_type_from_docstring can clearly get 
None, but it has a type of plain str. Fix the annotation to match the 
expectation. Now, when rebuilding Mypy, the error goes away and this 
now works without crashing, and can completely generate stubs.

Author: henryiii

mypy/stubdoc.py

@@ -339,7 +339,7 @@ def find_unique_signatures(sigs: Sequence[Sig]) -> List[Sig]:
     return sorted(result)
 
 
-def infer_prop_type_from_docstring(docstr: str) -> Optional[str]:
+def infer_prop_type_from_docstring(docstr: Optional[str]) -> Optional[str]:
     """Check for Google/Numpy style docstring type annotation for a property.
 
     The docstring has the format "<type>: <descriptions>".