Fix type error in (X/Y)PowGate._act_on_

[maffoo]

There was a type error due to the fact that the common superclass EigenGate does not define _act_on_. We can act on each gate instead of looping over them, which avoids the type error.

[smitsanghavi]

If we are changing this to avoid type: ignores, might as well fix the same pattern in YPowGate for consistency

[balopat]

This looks good - though I do like the list comprehension as it's a bit more python-esque.
An alternative way to keep it is to actually use the protocol type in the type definition:

                ops = [ZPowGate(), H]  # type: List[SupportsActOn]
                assert all(gate._act_on_(args) for gate in ops)

What do you think of this direction?

[maffoo]

@balopat, I added a helper function to call _act_on_ on a series of gates. The helper has a type annotation as you suggested, which takes care of the types.