py/objtype: Handle NotImplemented return from binary special methods.

NotImplemented means "try other fallbacks (like calling __rop__
instead of __op__) and if nothing works, raise TypeError". As
MicroPython doesn't implement any fallbacks, signal to raise
TypeError right away.

tests/basics/class_notimpl.py

@@ -0,0 +1,50 @@
+# Test that returning of NotImplemented from binary op methods leads to
+# TypeError.
+try:
+    NotImplemented
+except NameError:
+    print("SKIP")
+    raise SystemExit
+
+class C:
+    def __init__(self, value):
+        self.value = value
+
+    def __str__(self):
+        return "C(%s)" % self.value
+
+    def __add__(self, rhs):
+        print(self, '+', rhs)
+        return NotImplemented
+
+    def __sub__(self, rhs):
+        print(self, '-', rhs)
+        return NotImplemented
+
+    def __lt__(self, rhs):
+        print(self, '<', rhs)
+        return NotImplemented
+
+    def __neg__(self):
+        print('-', self)
+        return NotImplemented
+
+c = C(0)
+
+try:
+    c + 1
+except TypeError:
+    print("TypeError")
+
+try:
+    c - 2
+except TypeError:
+    print("TypeError")
+
+try:
+    c < 1
+except TypeError:
+    print("TypeError")
+
+# NotImplemented isn't handled specially in unary methods
+print(-c)