Use duck-typing to check for non-numeric types in approx()

[nicoddemus]

Fix #8132

[nicoddemus]

I did try type-annotating the return value like this:

Suggested change

	def __sub__(self, other):
	def __sub__(self, other: Any) -> Union[float, "Container"]:

However mypy complained with:

testing/python/approx.py:601:59: F821 undefined name 'Container'
testing/python/approx.py:608:60: F821 undefined name 'Container'

Is that because it is an inner class? Suggestions welcome. 👍

[jvansanten]

I think the Python number hierarchy is still useful in ApproxScalar, simply because anything that inherits from that hierarchy is guaranteed to be a scalar whose comparison methods return bool. ndarray and its many work-alikes (such as tf.DeviceArray) are not scalars, and their comparison methods return something that is only convertible to bool when it has a single element. In other cases, the bool conversion raises ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all().. I put in an alternate proposal in #8137 that unifies the treatment of ndarray and its work-alikes, which I think would be less surprising to users. Feedback welcome!

[nicoddemus]

Closing in favor of #8137.