test/py/tests/test_sleep.py: test time approximately

On qemu errors like
assert 2.999650001525879 >= 3
occur.

According to the comment in the code the test is meant to be
approximate. So we should accept some milliseconds less.

Cc: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>