py/modmath: Implement math.isclose() for non-complex numbers.

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.

tests/float/math_isclose.py

@@ -0,0 +1,47 @@
+# test math.isclose (appeared in Python 3.5)
+
+try:
+    from math import isclose
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+def test(a, b, **kwargs):
+    print(isclose(a, b, **kwargs))
+
+def test_combinations(a, b, **kwargs):
+    test(a, a, **kwargs)
+    test(a, b, **kwargs)
+    test(b, a, **kwargs)
+    test(b, b, **kwargs)
+
+# Special numbers
+test_combinations(float('nan'), 1)
+test_combinations(float('inf'), 1)
+test_combinations(float('-inf'), 1)
+
+# Equality
+test(1.0, 1.0, rel_tol=0.0, abs_tol=0.0)
+test(2.35e-100, 2.35e-100, rel_tol=0.0, abs_tol=0.0)
+test(2.1234e100, 2.1234e100, rel_tol=0.0, abs_tol=0.0)
+
+# Relative tolerance
+test(1000.0, 1001.0, rel_tol=1e-3)
+test(1000.0, 1001.0, rel_tol=1e-4)
+test(1000, 1001, rel_tol=1e-3)
+test(1000, 1001, rel_tol=1e-4)
+test_combinations(0, 1, rel_tol=1.0)
+
+# Absolute tolerance
+test(0.0, 1e-10, abs_tol=1e-10, rel_tol=0.1)
+test(0.0, 1e-10, abs_tol=0.0, rel_tol=0.1)
+
+# Bad parameters
+try:
+    isclose(0, 0, abs_tol=-1)
+except ValueError:
+    print('ValueError')
+try:
+    isclose(0, 0, rel_tol=-1)
+except ValueError:
+    print('ValueError')