I.e. nan is not considered close to any other value, including nan`. infand-inf`` are only considered close to themselves. See also PEP 485.

If that will be accepted, we also could use math.isclose() to implement almosteq() for the fp backend. Probably this should be slightly faster.

Right now we have:

>>> from mpmath import inf, mp
>>> mp.almosteq(inf, inf)
False

c.f.

>>> from math import inf, isclose
>>> isclose(inf, inf)
True

Are there good reasons why almosteq() behave differently from the math.isclose()?