Mitigate Python build directory naming conflict

On Mac and Windows, the default directory name conflicts with the BUILD
file we have.
diff --git a/.gitignore b/.gitignore
index cc70659..66c546e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,6 +5,7 @@
 objs
 
 # Python items
+python_build/
 .coverage*
 .eggs
 .tox