lpython.py: Support __bool__() for UnsignedInteger

Author: ubaidsk

src/runtime/lpython/lpython.py

@@ -26,6 +26,9 @@ def __init__(self, bit_width, value):
         self.bit_width = bit_width
         self.value = value
 
+    def __bool__(self):
+        return self.value != 0
+
     def __add__(self, other):
         if isinstance(other, self.__class__):
             return UnsignedInteger(self.bit_width, (self.value + other.value) % (2**self.bit_width))