stdlib/3/inspect: fix _ParameterKind being an empty enum (#4383)

This makes mypy think that conditions like

    parameter.kind is Parameter.POSITIONAL_OR_KEYWORD

are always false, which triggers `unreachable` warnings, among other
problems.

tests/stubtest_whitelists/py3_common.txt

@@ -210,6 +210,11 @@ importlib.machinery.WindowsRegistryFinder.find_spec
 importlib.util.spec_from_file_location
 importlib.util.spec_from_loader
 inspect.BoundArguments.__init__
+inspect.Parameter.KEYWORD_ONLY
+inspect.Parameter.POSITIONAL_ONLY
+inspect.Parameter.POSITIONAL_OR_KEYWORD
+inspect.Parameter.VAR_KEYWORD
+inspect.Parameter.VAR_POSITIONAL
 inspect.Parameter.replace
 inspect.Signature.replace
 inspect.getabsfile