py/objfloat: Fix handling of negative float to power of nan.

Prior to this commit, pow(-2, float('nan')) would return (nan+nanj), or
raise an exception on targets that don't support complex numbers.  This is
fixed to return simply nan, as CPython does.

Signed-off-by: Damien George <damien@micropython.org>

tests/float/inf_nan_arith.py

@@ -0,0 +1,20 @@
+# Test behaviour of inf and nan in basic float operations
+
+inf = float("inf")
+nan = float("nan")
+
+values = (-2, -1, 0, 1, 2, inf, nan)
+
+for x in values:
+    for y in values:
+        print(x, y)
+        print("  + - *", x + y, x - y, x * y)
+        try:
+            print("  /", x / y)
+        except ZeroDivisionError:
+            print("  / ZeroDivisionError")
+        try:
+            print("  ** pow", x ** y, pow(x, y))
+        except ZeroDivisionError:
+            print("  ** pow ZeroDivisionError")
+        print("  == != < <= > >=", x == y, x != y, x < y, x <= y, x > y, x >= y)