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
Almost all the drivers in the tree now opt out of the byte-compat
behavior. The default has been flipped, so we can remove that from
all those drivers and remove it from the PR template so people don't
have to think about it anymore.
