Tc-admin

This switches from using [patiencediff](https://pypi.org/project/patiencediff/) to calling ``diff`` with temporary files, for faster results on the large configuration files in [firefox-ci-tc](https://firefox-ci-tc.services.mozilla.com/).

This version alters the handling of `check_path`: where previously `tc-admin check` would change to this directory and run `pytest`, it now runs `pytest $check_path`. This results in check names containing the full path to the check, but has the advantage that the current directory is the same as for other `tc-admin` subcommands.

This version also normalizes `root_url()` to not end with a slash.

This version fixes an error where the Updater constructor was defined as an async function (182).

Changes:
* Add support for `appconfig.description_prefix` (thanks to bhushan-borole!)
* Many, many version bumps, notably to `taskcluster=~39.0.0`