Make BaseDensePauliString an Iterable class

[tanujkhattar]

Description of the issue
BaseDensePauliString already defines a __getitem__ method which works for ints, and hence is already implicitly an iterable. However, mypy doesn't recognize it as an Iterable unless an explicit __iter__ method is defined.

>>> import cirq
>>> from typing import Iterable
>>> m = cirq.DensePauliString("XYZ")
>>> print([p for p in m])
[cirq.X, cirq.Y, cirq.Z]
>>> isinstance(p, Iterable) # Should return true.
False 

We can easily define the __iter__ method as:

    def __iter__(self) -> Iterator['cirq.Gate']:
        for i in range(len(self)):
            yield self[i]

However, this currently breaks a test due to an unfortunate combination of two bugs: #4835 and #4834

Cirq version
0.14.0.dev

[tanujkhattar]

From cirq sync: +1, should be done.