Mitigate Python build directory naming conflict

On Mac and Windows, the default directory name conflicts with the BUILD
file we have.

.gitignore

@@ -5,6 +5,7 @@ libs
 objs
 
 # Python items
+python_build/
 .coverage*
 .eggs
 .tox

setup.cfg

@@ -3,6 +3,9 @@
 [coverage:run]
 plugins = Cython.Coverage
 
+[build]
+build_base=python_build
+
 [build_ext]
 inplace=1