Bot to handle pull request merging

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As stated in the section entitled

"`Document steps to commit a pull request`_", the desire is to

maintain a linear history for cpython. Unfortunately,

Github's [#github]_ web-based workflow does not support a linear

history. Because of this, a bot should be written to substitute for

GitHub's in-browser commit abilities.

To start, the bot should accept commands to commit a pull request

against a list of branches. This allows for committing a pull request

that fixes a bug in multiple versions of Python.

More advanced features such as a commit queue can come later. This

would linearly apply accepted pull requests and verify that the

commits did not interfere with each other by running the test suite

and backing out commits if the test run failed. To help facilitate

the speed of testing, all patches committed since the last test run

can be applied and run in a single test run as the optimistic

assumption is that the patches will work in tandem.

Inspiration or basis of the bot could be taken from pre-existig bots

such as Homu [#homu]_ or Zuul [#zuul]_.