This commit sets the line length to 120 and adds some files to exclude
from processing. The exclusions are pretty standard.

This commit adds configuration options to turn off detection of
namespace packages, since we're not using them, and to explicitly
specify the src/ directory as the package root.

In this commit I'm adding a configuration section for ruff that sets
the line length to 120 (same as was done for black), lists the src
directory as the package root relative to which imports should be
resolved, and sets Python 3.6 as the minimum target version.

I chose to put the configuration in pyproject.toml instead of a separate
file because it keeps the two line-length settings (for ruff and black)
close together and hopefully makes it harder to desynchronize them if we
change the line length later.

This ensures that black will run successfully on the existing files.

This gets all the tests invoked by tox to pass.