Add spy_return_iter attribute to spy

[vokracko]

Recently I've run into situation where I wanted to spy on a method returning Iterator.
That wasn't possible because the iterator is already consumed by the time you try to assert it.

In this PR I add a new attribute, spy_return_iter storing a duplicate of the returned iterator (leveraging itertools.tee).

[nicoddemus]

Thanks for the contribution @vokracko!

Left some minor comments, please take a look.

[nicoddemus]

Does this work?

Suggested change

	assert spy.spy_return_list == [ANY]
	assert isinstance(spy.spy_return_list[0], Iterator)

[vokracko]

Added in e6c26b2

[nicoddemus]

Minor suggestion to make this clearer:

Suggested change

	def test_spy_return_iter_unset_in_last_call(mocker: MockerFixture) -> None:
	class Foo:
	iterables = [
	(i for i in range(3)),
	[3, 4, 5],
	]
	def bar(self) -> Iterable[int]:
	return self.iterables.pop(0)
	foo = Foo()
	spy = mocker.spy(foo, "bar")
	result_iterator = list(foo.bar())
	assert result_iterator == [0, 1, 2]
	assert list(spy.spy_return_iter) == result_iterator
	result_iterable = foo.bar()
	assert result_iterable == [3, 4, 5]
	assert spy.spy_return_iter is None
	def test_spy_return_iter_resets(mocker: MockerFixture) -> None:
	class Foo:
	iterables = [
	(i for i in range(3)),
	99,
	]
	def bar(self) -> Iterable[int]:
	return self.iterables.pop(0)
	foo = Foo()
	spy = mocker.spy(foo, "bar")
	result_iterator = list(foo.bar())
	assert result_iterator == [0, 1, 2]
	assert list(spy.spy_return_iter) == result_iterator
	assert foo.bar() == 99
	assert spy.spy_return_iter is None

Mixing iterator and iterables on this test is not necessary for the logic and adds a small bit of confusion.