py/modmath: Add math.tau, math.nan and math.inf constants.

Configurable by the new MICROPY_PY_MATH_CONSTANTS option.

tests/float/math_constants.py

@@ -0,0 +1,11 @@
+# Tests various constants of the math module.
+try:
+    import math
+    from math import exp, cos
+except ImportError:
+    print("SKIP")
+    raise SystemExit
+
+print(math.e == exp(1.0))
+
+print(cos(math.pi))

tests/float/math_constants_extra.py

@@ -0,0 +1,17 @@
+# Tests constants of the math module available only with MICROPY_PY_MATH_CONSTANTS.
+try:
+    import math
+    from math import isnan
+
+    math.tau
+except (ImportError, AttributeError):
+    print("SKIP")
+    raise SystemExit
+
+print(math.tau == 2.0 * math.pi)
+
+print(math.inf == float("inf"))
+print(-math.inf == -float("inf"))
+
+print(isnan(math.nan))
+print(isnan(-math.nan))