Shouldn’t the inheritance be the other way round? In the MRO foo currently comes before bar_for_foo_mixin, and then rightfully complains. With class myfoo(bar_for_foo_mixin, foo) it should work.
And I am not sure if your class design is the right way to do it. Since you use a mixin for implementing bar it might be better not to derive from foo and just register it with the ‘foo’ class (i.e. foo.register(myfoo)). But this is just my gut feeling.
For completeness, here is the documentation for ABCs.