`pytest.approx` fails with `TypeError` when testing approximate value of a `decimal.Decimal`

description

When testing the approximate value of a decimal.Decimal the following TypeError occurrs
TypeError: unsupported operand type(s) for -: 'float' and 'decimal.Decimal'

I would expect pytest.approx to test for approximate equality with a floating point value without having to either convert the decimal to a float or testing explicitly against another decimal.

pip list

Package      Version
------------ -------
atomicwrites 1.4.0
attrs        20.3.0
colorama     0.4.4
iniconfig    1.1.1
packaging    20.9
pip          21.0.1
pluggy       0.13.1
py           1.10.0
pyparsing    2.4.7
pytest       6.2.2
setuptools   54.2.0
toml         0.10.2

pytest and os versions

pytest: 6.2.2
os: windows 10

minimal example

import decimal
import pytest
import math

3.14159 == pytest.approx(math.pi)  # works
decimal.Decimal('3.14159') == pytest.approx(math.pi)  # raises TypeError

Start at the pytest.approx entry point and reproduce the minimal Decimal/float example from the issue. Trace the approximate-comparison path and add regression coverage for that example; done means Decimal compared with a floating-point value no longer raises TypeError and still checks approximate equality.