Runcommands

- Drop support for Python 3.5 since it's no longer supported.
- Add support for Python 3.9. For now, this just means adding Python 3.9
to the list of supported version and the list of versions tested via
tox.
- When possible, set type from default value for container args that
don't specifiy a type.

- Improved/fixed/standardized how command results are processed. A
`Result` object is now created in most cases, except when a command
returns something that's not `None`, an `int`, or a `Result`.

When a command returns `None` or an `int`, 0 or the `int` is now
returned from `Command.console_script` instead of 1 (not sure what the
rationale behind returning 1 was in these cases).

Callbacks will now receive `Result` objects in most cases, which gives
them access to commands' stdout, etc.

- Fixed/improved callback handling. See 58be7ab86a26 for details.
- In `Printer`, moved default stream assignments for the various print
methods from keyword args into the method body. This allows
`sys.stdout` and `sys.stderr` to be redirected using
`contextlib.redirect_stdout` in tests.

- Made `data` property of `Command` instances read only.
- More args are now passed to command callbacks:
- `Command` instance (as before)
- result of running command
- `aborted` flag, which will be the same for *all* callbacks; this is
intended for use in base command callbacks for deciding how to do
cleanup

- Added 1.0a57 change log.

- Added `is_type()` utility function and used it to tidy up code in a couple
places.

- Added `Parameter` class to wrap `inspect.Parameter`. It has some convenience
functions that allows some code to be tidied.

- Fixed `Command.find_parameter()` so that it does what it says it does. In
particular, it now tries to find a parameter with the passed-in name before
normalizing the name and trying to find an _arg_ with that name (and then
retrieving the parameter from that arg). This allows parameters that don't
have a corresponding arg to be retrieved.

- Improved how args are passed down from base commands to subcommands.

The main change is that a given base arg will now be passed down to a
subcommand when it has a corresponding function _parameter_ rather than a
corresponding _arg_ as before (parameter = function parameter; arg = command
line arg).

This allows subcommands to have required keyword-only parameters that can
receive args from a base command. Since required keyword-only parameters
don't correspond to command line args, this forces these args to be passed to
the base command while still giving subcommands a way to get access to them
when needed.

The use case for this is args that need to be handled consistently at a
higher level. The specific use case that motivated this was handling "dry
run" functionality (esp. pertaining to database transactions).

- Constrained flake8 to <3.8 for now, since 3.8 is finding "errors" that aren't
errors--it's calling out quoted strings as undefined names in function arg
annotations 🤔

- Fixed `Data` class used for attaching arbitrary data to commands via `data`
attribute on `Command` instances (added in 1.0a55). Accessing the internal
data dict was causing infinite recursion.
- Improved `Data` so that when a `dict` is added it will be converted to
a `Data` object. This is for convenience and consistent access to nested
data.
- Modified `Printer` to flush by default when the console is a TTY. This aligns
with what I assume is usually the desired behavior when running commands
interactively.