I noticed that this question specifically asked about py.test. py.test 3.0 includes an approx() function (well, really class) that is very useful for this purpose.
import pytest
assert 2.2 == pytest.approx(2.3)
# fails, default is ± 2.3e-06
assert 2.2 == pytest.approx(2.3, 0.1)
# passes
# also works the other way, in case you were worried:
assert pytest.approx(2.3, 0.1) == 2.2
# passes
The documentation is here.