Runcommands

In progress...

- Added support for Python 3.10 and 3.11. Initially, this just means
testing against these versions when creating a new release.
- Dropped _official_ support for Python 3.6 since it has been
end-of-life for about a year now. Note that RunCommands can still be
installed on 3.6, but 3.6 is no longer tested against when creating
a new release.
- Relaxed dependency constraints to `>=x` rather `>=x.y,<x+1`. This
allows compatibility with more projects.
- Fixed a bug in the printer utility that would throw an exception if
_no_ args were passed to any of the print functions (e.g., using
`printer.print()` to print a blank line).
- Updated the styling of headers printed via `printer.header()` in order
to make headers more visually prominent.

- Allow default arg values to be specified via environment variables
like so: `some_arg: arg(envvar="SOME_ENV_VAR") = None`. If no value is
passed for `some_arg` on the command line, the value of `SOME_ENV_VAR`
will be used, if it's defined.

This is an alternative to using a config file to define default arg
values. It's probably more useful for standalone console scripts
versus collections of commands. Both types of defaults can be used at
the same time, although this isn't recommended (env var defaults take
precedence over config file defaults).

- Make some printer methods print to stderr by default: `debug`,
`error`, `warning`. In the switch to Rich in 1.0a67, these methods
were changed inadvertently to print to stdout.
- Fixed potential infinite loop when finding project root.
- Fixed infinite recursion when attempting to find a commands module in
a directory that didn't contain one (or any of its parents).

- Fix a couple small issues with the switch to Rich in 1.0a67.

- Use [Rich](https://pypi.org/project/rich/) for color printing and
other terminal formatting. Rich is cross platform and opens up some
new options for better formatting.