Make EqualsTester notice incorrect delegation

[dabacon]

This extends EqualsTester to notice when someone incorrectly does not return NotImplemented in __eq__ for objects of an unrecognized type. Doing this can cause commutativity to fail when implementing other objects that should be equal to this object. (It was worse in python 2, where depending on whether one inherited from object or not could cause this).

This also fixes places where Cirq code was doing this incorrectly.

Fixes #877

One final note is that the eigen_gate_test was changed because apparently Sympy does this incorrectly

>>> import sympy
>>> s = sympy.Symbol("a")
>>> class MyClass:
...   def __eq__(self, other):
...     return True
... 
>>> s.__eq__(MyClass())
False

[dabacon]

Cirq is secretly just a test of places where Sympy fails to be pythonic?

[balopat]

I just reran your checks as well on this PR to see the difference, couldn't repro the lint error yet on this branch. And now it passed. Very odd. XRef #3716.

[dabacon]

@Strilanc probably the best one to look at this sense he wrote the original definition of the problem (and there seems to be a slight difference in what he said, since we don't need a fallback to reference checking to get this work).

[dabacon]

@balopat this could use a triage to a reviewer

[95-martin-orion]

The comments in the test here are incredibly helpful for parsing it, thanks!

One nit and this is good to go.

[dabacon]

tx @95-martin-orion !