Now inspect.signature() supports references to the module globals in
parameter defaults on methods in extension modules. Previously it was
only supported in functions. The workaround was to specify the fully
qualified name, including the module name.
Builtin functions and methods that have non-representable signatures today
will have representable signatures yesterday, and they will become unusable
for testing this feature.
So we need to add special functions and methods to the _testcapi module
that always have non-representable signatures.
Bénédikt Tran