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
diff --git a/setup.cfg b/setup.cfg
index add6ee8..7194716 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -3,6 +3,9 @@
 [coverage:run]
 plugins = Cython.Coverage
 
+[build]
+build_base=python_build
+
 [build_ext]
 inplace=1