Fix event.custom_event first call

[MightyJosip]

This should fix #2070.

In pygame 2.0.0.dev3 (#1151) return value used to be:

int _custom_event = PGE_USEREVENT + 1;
    int result = _custom_event + 1;
    return PyInt_FromLong(result);

So at the end if default user event is 10, 12 would be returned. To fix this (it should return 11) the following was done in dev4 (#1275)

int _custom_event = PGE_USEREVENT;
   return PyInt_FromLong(_custom_event++);

It updated custom event value only AFTER returning it. Fix could be done even with replacing _custom_event++ with _custom_event++, but I decided to go with returning the old initialization of _custom_event

[MyreMylar]

Looks good to me 👍 🎉