Bump major version (3.0.0 -> 4.0.0)

Rationale: Breaking changes in scope handling

* Request handling is sped up by processing API config once, at
  application start, rather than on every request. Additionally, we
  now avoid an unnecessary Python->JSON->Python roundtrip.
* Dead code from the JSON-RPC support has been removed.
* The api_backend and api_backend_service modules have been removed;
  nearly all of that code was dead anyway.
* The OAuth scope support now properly supports situations where
  multiple scopes must be present for a request to proceed.
* Added a deprecation warning when auth_level is used.

endpoints/__init__.py

@@ -33,4 +33,4 @@
 from .users_id_token import InvalidGetUserCall
 from .users_id_token import SKIP_CLIENT_ID_CHECK
 
-__version__ = '3.0.0'
+__version__ = '4.0.0'