Add a script to update the AUR packages

Summary:
For each AUR package, this script:

1. Completes/generates the package files
2. Applies the changes to the upstream git repository
3. Runs a full package build from the updated files
4. Commit the changes and push to upstream

It requires an archlinux environment to run (with all the tools
installed) and a git user with write permission to the upstream
directory.

The sha256 checksums are removed from the PKGBUILD file as they are
regenerated by the script to reduce maintenance on the file.

The python package was missing as a dependency and would cause the
build to fail if not already installed on the user machine.

A README is added to explain how to update the packages using the
script.

Depends on D7260.

Test Plan:
Ran the build in an archlinux container and checked everything worked
excepted the final push.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7261