Library to extend Python argparse stdlib with easy handling of subcommands

Extends argparse.ArgumentParser with a facility for configuring subcommands by convention. Each subcommand lives in its separate Python module.
The name of the subcommand is the module name (without superpackage names) with underscore replaced by dash.

To be a subcommand module, a module must have

import argparse_subcommand as ap_sub

meaning = "some help text for the subcommand"
def add_arguments(parser: ap_sub.ArgumentParser): ...  # configure the subcommand's sub-argparser
def execute(args: ap_sub.Namespace): ...  # run the subcommand

The module can also optionally have:

aliases = ["subcmd-alias1", "subcmd-alias2"]  # optional.

for making available the same subcommand under one or more alternative names (e.g. an abbreviation).

For use, create the parser as usual and then call the submodule scanner:

parser = ap_sub.ArgumentParser(epilog=explanation)
parser.scan("mysubcmds.subcmd1", "mysubcmds.subcmd2")  # or provide module object instead of str
args = parser.parse_args()
parser.execute_subcommand(args)  # or supply nothing, then parse_args() will be called internally

By convention, the subcommand modules (and only they) all go into a common package. If you do that, you can scan them all at once:

parser.scan("mysubcmds.*")

argparse_subcommand uses only one sub-parser group, so that subcommands cannot be nested, there is only one level of subcommands. It will execute importlib.import_module() on all modules mentioned in a scan() call as strings.
Multiple calls to scan() are allowed, each can have one or more arguments.
scan(..., strict=True) will exit when encountering a non-subcommand-module.
scan(..., trace=True) produces output helpful for debugging your subcommands setup.

That's all.

• 1.0, 2023-05:
  First release
• 1.1, 2024-01:
  Error messages and trace messages go to stderr, not stdout.
  More informative messages when scanned modules are non-subcommand modules. Small improvements to documentation.
  Added version history.
• ...