Add api_core package (#4210)

* Add api_core package

* Address review comments
diff --git a/MANIFEST.in b/MANIFEST.in
new file mode 100644
index 0000000..1fbc0d0
--- /dev/null
+++ b/MANIFEST.in
@@ -0,0 +1,3 @@
+include README.rst LICENSE
+recursive-include tests *
+global-exclude *.pyc __pycache__