Caused by rBa5c59fb90ef9.
Since Group Input and Output sockets happen to be of type `SOCK_CUSTOM`
[and since rBa5c59fb90ef9 custom py defined sockets are too :)] a check
introduced in rB513066e8ad6f that prevents connections for `SOCK_CUSTOM`
triggered.
Now refine the check, so it specifically looks for NODE_GROUP_INPUT /
NODE_GROUP_OUTPUT, too (this keeps the intention intact to not connect
group inputs to group outputs and vice versa, but allows custom py
defined sockets to connect again) and put it in new utility function.
Maniphest Tasks: T94827
Differential Revision: https://developer.blender.org/D13817
85df7036f7