As per PEP 485, this function appeared in for Python 3.5. Configured via
MICROPY_PY_MATH_ISCLOSE which is disabled by default, but enabled for the
ports which already have MICROPY_PY_MATH_SPECIAL_FUNCTIONS enabled.
In this port JavaScript is the underlying "machine" and MicroPython is
transmuted into JavaScript by Emscripten. MicroPython can then run under
Node.js or in the browser.