py: Change MP_UNARY_OP_INT to MP_UNARY_OP_INT_MAYBE.

To be consistent with MP_UNARY_OP_INT_FLOAT and MP_UNARY_OP_INT_COMPLEX,
and allow int() to first check if a type supports __int__ before trying
other things (as per CPython).

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

tests/basics/special_methods_intbig.py

@@ -0,0 +1,8 @@
+# Test class special methods, that use a bigint.
+
+class A:
+    def __int__(self):
+        return 1 << 100
+
+
+print(int(A()))