Add methods to api_core used by new autogenerator. (#6267)

* Add dispatch and deserialize methods.

This adds convenience methods used by client libraries produced by
gapic-generator-python.
* Mark test as Python 3 only.

* Fix import order.

* Address @theacodes feedback.

* Move dispatch to gapic_v2; alias remaining gapic_v1 modules.

* Fix import order.
5 files changed