Cherry-picker

- Change the calls to `subprocess` to use lists instead of strings. This fixes
the bug that affects users in Windows
([PR 238](https://github.com/python/core-workflow/pull/238)).

- Add `fix_commit_msg` configuration item. Setting fix_commit_msg to `true`
will replace the issue number in the commit message, from `` to `GH-`.
This is the default behavior for CPython. Other projects can opt out by
setting it to `false` ([PR 233](https://github.com/python/core-workflow/pull/233)
and [aiohttp issue 2853](https://github.com/aio-libs/aiohttp/issues/2853)).

- Support configuration file by using `--config-path` option, or by adding
`.cherry-picker.toml` file to the root of the project
([Issue 225](https://github.com/python/core-workflow/issues/225))