This patch moves the oauth1 abstract base class out of core and into
backends/base.py
This removes dependencies where backend code references code in the
core. The reasoning being that the core should know about the backend
interface, but the backends should not know anything about the core
(separation of concerns). And part of the risk here is a potential for
circular dependencies.
Partial-Bug: #1563101
Change-Id: I730798c71a6e8be8f32edd5bddb9e86cabd6ff65
a9d2daace2