tooltool client location, duplication and usage

Hey, the doc says

If you want to use the client, just copy out tooltool.py -- it has no dependencies.

I think it's a problem.

Having people copy that script here and there has two major disadvantages:

• it leads to some issues of having deprecated tooltool.py instances in many places.
• it makes it hard to deploy a new improved version
• we don't know if the version we use is the right one

For example the version in this repo says version == '1' but the one in mozharness
has the same version but is a different module.

We bumped into the issue because we started to work on https://phabricator.services.mozilla.com/D54090 to add a small retry mechanism to try
to reduce intermittents on Raptor, and decided to use mozbuild's tooltool version as
the source of truth, but it turns out this repo is the one :)

there's also https://github.com/mozilla/build-relengapi ?

I would like to make this proposal:

• ask people not to copy around tooltool.py
• remove all tooltool.py instances in mozilla-central
• release on pypi and our internal pypi a tooltool client with no dep that can be used in all py versions
• make that package a commodity everywhere in our CI and tests environements

Start by comparing this repository's tooltool.py with mozharness/client/tooltool.py and the mozbuild version mentioned in the issue. Trace where copied tooltool.py instances are used in mozilla-central and CI, then assess the PyPI and internal PyPI packaging requirements. Done means there is an agreed single distributable client and a migration plan for existing copies.