Include "autopage" in the name so that there's no possibility of future
colisions with argparse.

Clients that implement their own Actions need an API to allow them to
set the ColorHelpFormatter into color mode.

These are not helpful for distro packaging.

This is useful for tests in distro packaging.

Also ensure that we use setuptools>=43 so that pyproject.toml will
always be included as well.

The --exclude CLI option does not exist in the 0.7xx versions.

It is not enough to catch and handle KeyboardInterrupt while waiting for
the user to exit the pager. A KeyboardInterrupt can occur between any
two Python opcodes. Furthermore, it may (indeed, in some cases it
appears quite likely to) occur while flushing the write buffer and
closing the output stream.

The only way to fully protect against this is to ignore the signal
altogether for the duration of the cleanup.

If the output consumer closes before we are able to write all output to
it, then it is irrelevant whether the code inside the context manager
encountered a BrokenPipeError or if it only occurs when trying to flush
the buffer filled by that code. Therefore, set the SIGPIPE return code
if flushing the buffer fails.

Constants are available in the signal module, and may theoretically be
different on different platforms.

If the stream is closed, don't fail until we try to actually use it.

Add a function to create an AutoPager with the recommended settings for
help output, for use by programs that don't use the default argparse
help action.

By default, Python enables line buffering on stdout whenever stdout is
connected to a console - which is exactly when we will start a pager. As
a consequence, the --quit-if-one-screen option was never enabled unless
the AutoPager was explicitly created with line_buffering=False.

This isn't a great default, given that the programmer knows that the
output will be going to a pager, so only disable --quit-if-one-screen if
line buffering is explicitly turned on.

Split parts that would be common to both regular and asynchronous
context managers into separate methods.

Explicitly set line buffering to False when printing the help, to ensure
that we always quit the pager if there is less than one screen's worth
of data.

Also explicitly set allow_color to True, since we're now relying on
this.

Start introducing styling in the help output from argparse. For now this
is just underlining section titles and putting metavars in italics.

Set the correct environment for when lv is specified as the pager.

If we can't start the pager (e.g. because less or the pager specified as
$PAGER doesn't exist on the system), catch the exception and fall back
to outputting directly to the terminal without paging.

Previously, we always allowed less to reset the terminal when exiting
(as it does by default when being run manually), so that the output of
the program did not remain on screen. This is desirable in some
applications, but less so in others. Git does not clear the screen and
instead leaves the last shown output displayed. Change the default to do
the same here.

Since clearing the screen is sometimes desirable, add a configuration
option to allow the developer to choose this.

If we quit the pager immediately due to having less than one screen's
worth of data to display, then the output will obviously always be left
on the screen. This is inconsistent with a request to reset the terminal
on completion, so do not pass this option when the terminal is to be
reset.

If the user wishes to override the settings configured in the
environment, they need to be able to specify a custom PAGER environment
variable containing arguments as well as the name of the pager. Popen
does not support this being passed as a string unless shell=True, but we
generally don't want to be enabling shell interpretation. Therefore
split the arguments using the shlex module.

Convert the 'errors' argument to an ErrorStrategy, so that if a valid
strategy is passed as a string it will still work when we get
self._set_errors.value, and if an invalid strategy is passed we'll get
an exception raised immediately.

The string value of an Enum member is a representation of the name, not
the actual value.