Make cherry picker as a script

[louisom]

Relate to #64 #69

I'm not sure if this change will break other bot or something.

This commit make cherry-picker more easier to use, you don't need to let cpython folder under cherry_picker anymore.

Example:

$ ls
cpython   # Assume we already clone cpython in our dev dir and upstream setting
$ git clone https://github.com/python/core-workflow.git
$ cd core-workflow
$ python -m venv venv
$ source venv/bin/activate
(venv) $ python -m pip install -r cherry_picker/requirements.txt
(venv) $ cp -r cherry_picker venv/lib/python3.6/site-packages/
(venv) $ cd ..
(venv) $ python -m cherry_picker deadbeaf 3.6

[Mariatta]

Thanks @lulouie :)