Add TODO: provide the functionality in previous versions of api_core

We have to add this functionality manually if the user has not
upgraded to this version of api_copre

Author: vchudnov-g

google/api_core/__init__.py

@@ -23,6 +23,10 @@
 
 __version__ = api_core_version.__version__
 
+# TODO: Until dependent artifacts require this version of
+# google.api_core, the functionality below must be made available
+# manually in those artifacts.
+
 check_python_version = _python_version_support.check_python_version
 check_python_version(package="google.api_core")