abc: Add ABC base class.

This trivial addition will allow less code differences between standard
python classes and micropython code.

Signed-off-by: Alon Bar-Lev <alon.barlev@gmail.com>

python-stdlib/abc/abc.py

@@ -1,2 +1,6 @@
+class ABC:
+    pass
+
+
 def abstractmethod(f):
     return f