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

python-stdlib/abc/manifest.py

@@ -1,3 +1,3 @@
-metadata(version="0.0.1")
+metadata(version="0.1.0")
 
 module("abc.py")