4.0: remove the deprecated internal= alias and enable_self_transition_entries

Why

APIs deprecated during 3.x in favor of transition kinds (#723) are removed in the next major.

Remove the internal= transition argument and the native internal key; kind (Transition kinds: kind=external|local|internal across Python, YAML and SCXML #723) is the only spelling.
Remove enable_self_transition_entries (deprecated in fix: internal transitions enter and exit the right states #696).
Remove the atomic-source special case from get_transition_domain, leaving the three branches of Transition kinds: kind=external|local|internal across Python, YAML and SCXML #723.
Upgrade page "3.x to 4.0" lists each removal with its replacement.

Add here any other 3.x deprecation that should be removed in 4.0.