Interface checker doesn't handle mixins correctly

In `pyface` the following pattern is common:

```
import traits.has_traits
traits.has_traits.CHECK_INTERFACES = 2

from traits.api import Interface, HasTraits, provides

class ITest(Interface):

def foo(self):
pass

class MTest(object):

def foo(self):
pass

@provides(ITest)
class Test(HasTraits, MTest):
pass

t = Test()
t.foo()
```

This fails with:

```
InterfaceError: The 'Test' class does not implement the 'foo' method of the 'ITest' interface.
```

even though there is a `foo` method provided by the `MTest` instance.

See https://github.com/enthought/pyface/issues/102