Fix stubgen crash when module doesn't set __spec__ (#651)

Per https://docs.python.org/3/library/importlib.html#importlib.util.find_spec,
find_spec() can throw ValueError if __spec__ isn't set.

src/stubgen.py

@@ -641,7 +641,7 @@ def process_general(m: Match[str]) -> str:
             def is_valid_module(module_name: str) -> bool:
                 try:
                     return importlib.util.find_spec(module_name) is not None
-                except ModuleNotFoundError:
+                except (ModuleNotFoundError, ValueError):
                     return False
 
             full_name, mod_name, cls_name = m.group(0), m.group(1)[:-1], m.group(2)