Add ModuleNotFoundError for Python >= 3.6 (#1083)

https://github.com/python/cpython/blob/6f0eb93183519024cb360162bdd81b9faec97ba6/Objects/exceptions.c#L707
https://docs.python.org/3/library/exceptions.html#ModuleNotFoundError

stdlib/3/builtins.pyi

@@ -904,6 +904,8 @@ class EOFError(Exception): ...
 class FloatingPointError(ArithmeticError): ...
 class IOError(EnvironmentError): ...
 class ImportError(Exception): ...
+if sys.version_info >= (3, 6):
+    class ModuleNotFoundError(ImportError): ...
 class IndexError(LookupError): ...
 class KeyError(LookupError): ...
 class MemoryError(Exception): ...