**- Why I did it**
We were building a custom version of Supervisor because I had added patches to prevent hangs and crashes if the system clock ever rolled backward. Those changes were merged into the upstream Supervisor repo as of version 3.4.0 (http://supervisord.org/changes.html#id9), therefore, we should be able to simply install the vanilla package via pip. This will also allow us to easily move to Python 3, as Python 3 support was added in version 4.0.0.
**- How I did it**
- Remove Makefiles and patches for building supervisor package from source
- Install Python 3 supervisor package version 4.2.1 in Buster base container
- Also install Python 3 version of supervisord-dependent-startup in Buster base container
- Debian package installed binary in `/usr/bin/`, but pip package installs in `/usr/local/bin/`, so rather than update all absolute paths, I changed all references to simply call `supervisord` and let the system PATH find the executable to prevent future need for changes just in case we ever need to switch back to build a Debian package, then we won't need to modify these again.
- Install Python 2 supervisor package >= 3.4.0 in Stretch and Jessie base containers
We were building our own python-click package because we needed features/bug fixes available as of version 7.0.0, but the most recent version available from Debian was in the 6.x range.
"Click" is needed for building/testing and installing sonic-utilities. Now that we are building sonic-utilities as a wheel, with Click specified as a dependency in the setup.py file, setuptools will install a more recent version of Click in the sonic-slave-buster container when building the package, and pip will install a more recent version of Click in the host OS of SONiC when installing the sonic-utilities package. Also, we don't need to worry about installing the Python 2 or 3 version of the package, as the proper one will be installed as necessary.
This is an addendum to #3958, which also instructs apt to ignore the "Valid Until" date in Release files inside the slave containers, making a complete solution, much like the previously abandoned PR #2609. This patch also unifies file names and contents.
When the Debian team archives a repo, it stops updating the "Valid Until" date, thus apt-get will not apply updates for that repo unless we explicitly tell it to ignore the "Valid Until" date. Also, this has become an issue with active (i.e., non-archived) repos twice in the past year because the Debian folks seem to occasionally let the expiration lapse before updating the date. This will cause SONiC builds to fail with a message like E: Release file for http://debian-archive.trafficmanager.net/debian-security/dists/jessie/updates/InRelease is expired (invalid since 3d 3h 11min 20s). Updates for this repository will not be applied. until the dates have been updated and propagated to all mirrors. With this patch, SONiC should no longer be affected by lapsed "Valid Until" dates, whether they be accidental or purposeful.
