Bump post version number

Author: tlestang

setup.py

@@ -164,7 +164,7 @@ def compile_KLU():
 
 setup(
     name="pybamm",
-    version=load_version() + ".post2",
+    version=load_version() + ".post3",
     description="Python Battery Mathematical Modelling.",
     long_description=readme,
     long_description_content_type="text/markdown",