approx: Better handle Decimal in sequences and mappings

[pmourlanne]

What's the problem this feature will solve?

Right now approx handles Decimal comparisons gracefully, thanks to #3247. We can do this:

class TestApprox:
    ...

    def test_decimal(self):
        within_1e6 = [
            (Decimal("1.000001"), Decimal("1.0")),
            (Decimal("-1.000001"), Decimal("-1.0")),
        ]
        for a, x in within_1e6:
            assert a == approx(x)

approx also knows how to handle sequences and mappings:

class TestApprox:
    ...

    def test_list(self):
        actual = [1 + 1e-7, 2 + 1e-8]
        expected = [1, 2]

        # Return false if any element is outside the tolerance.
        assert actual == approx(expected, rel=5e-7, abs=0)
        assert actual != approx(expected, rel=5e-8, abs=0)
        assert approx(expected, rel=5e-7, abs=0) == actual
        assert approx(expected, rel=5e-8, abs=0) != actual

    ...

    def test_dict(self):
        actual = {"a": 1 + 1e-7, "b": 2 + 1e-8}
        # Dictionaries became ordered in python3.6, so switch up the order here
        # to make sure it doesn't matter.
        expected = {"b": 2, "a": 1}

        # Return false if any element is outside the tolerance.
        assert actual == approx(expected, rel=5e-7, abs=0)
        assert actual != approx(expected, rel=5e-8, abs=0)
        assert approx(expected, rel=5e-7, abs=0) == actual
        assert approx(expected, rel=5e-8, abs=0) != actual

approx doesn't handle Decimal within sequences and mappings:

class TestApprox:
    ...

    def test_list_decimal(self):
        actual = [Decimal("1.000001"), Decimal("2.000001")]
        expected = [Decimal("1"), Decimal("2")]

        assert actual == approx(expected)

    ...

    def test_dict_decimal(self):
        actual = {"a": Decimal("1.000001"), "b": Decimal("2.000001")}
        # Dictionaries became ordered in python3.6, so switch up the order here
        # to make sure it doesn't matter.
        expected = {"b": Decimal("2"), "a": Decimal("1")}

        assert actual == approx(expected)

Both of these tests fail with TypeError: unsupported operand type(s) for *: 'float' and 'decimal.Decimal'

Describe the solution you'd like

I would like these kind of tests to be passing :) A linked PR should be following shortly 👀

Alternative Solutions

Additional context