Remove python3 test progress tracking

The share of python3-uncompliant drivers has dropped to such a small
fraction that I've forgotten to enforce that new drivers be added to
the matrix.

So, at this point, it's time to flip the whitelist to a blacklist and
just mark the drivers that haven't been tested. This removes the
matrix and generation script, content from the makesupported target,
and instructions from the PR template. It replaces those with a simple
list of drivers in tests/py3-untested-drivers.txt

.github/pull_request_template.md

@@ -7,6 +7,6 @@ The following must be true before PRs can be merged:
 1. Commits in a single PR should be related. Squash intermediate commits into logical units (i.e. "fix tests" commits need not survive on their own). Keep cleanup commits separate from functional changes.
 1. Major new features or bug fixes should reference a [CHIRP issue](https://chirpmyradio.com/projects/chirp/issues) _in the commit message_. Do this with the pattern `Fixes #1234` or `Related to #1234` so that the ticket system links the commit to the issue.
 1. Please write a reasonable commit message, especially if making some change that isn't totally obvious (such as adding a new model, adding a feature, etc). The first line of every commit is emailed to the users' list after each build. It should be short, but meaningful for regular users (examples: "thd74: Fixed tone decoding" or "uv5r: Added settings support").
-1. New drivers should be accompanied by a test image in `tests/images` (except for thin aliases where the driver is sufficiently tested already). All new drivers must use `MemoryMapBytes`. New drivers and radio models will affect the Python3 test matrix. You should regenerate this file with `tox -emakesupported` and include it in your commit.
+1. New drivers should be accompanied by a test image in `tests/images` (except for thin aliases where the driver is sufficiently tested already). All new drivers must use `MemoryMapBytes`.
 1. All files must be GPLv3 licensed or contain no license verbiage. No additional restrictions can be placed on the usage (i.e. such as noncommercial).
 1. Do not add new py2-compatibility code (No new uses of `six`, `future`, etc).

README.md

@@ -3,7 +3,5 @@
 This is the official git repository for the
 __[CHIRP](https://www.chirpmyradio.com)__ project.
 
-When submitting PRs, please squash commits into logical units in a fashion
-similar to what is currently expected by the project on the mailing list.
-
-For Python3 driver testing status, see [this file](tests/Python3_Driver_Testing.md).
+When submitting PRs, please see [this file](.github/pull-request-template.md)
+for rules and guidelines.