Any way to make argparse not print "actions: <action>" here?

Consider this skeleton of a script that uses argparse.

import argparse
ap = argparse.ArgumentParser()
sp = ap.add_subparsers(
    required=True,
    metavar="<action>",
    title="actions"
)
sp.add_parser("alpha", help="action one")
sp.add_parser("beta", help="action two")
ap.parse_args()

When invoked with no arguments, the output is acceptable.

$ python3 test.py
usage: test.py [-h] <action> ...
test.py: error: the following arguments are required: <action>

However, the --help output looks silly:

$ python3 test.py --help
usage: test.py [-h] <action> ...

options:
  -h, --help  show this help message and exit

actions:
  <action>
    alpha     action one
    beta      action two

How can I get the --help output to look like this instead?

$ python3 test.py --help
usage: test.py [-h] <action> ...

options:
  -h, --help  show this help message and exit

actions:
    alpha     action one
    beta      action two

I’m reading argparse.HelpFormatter._format_action(), and I don’t see a way to suppress the line with the “action_header” and help after the actions: heading, since the action_header is the list of options/args by default or the metavar if one is given. It’s possible to give a different help formatter, like ArgumentParser(formatter_class=MyHelpFormatter()), but sadly it doesn’t seem easy to customize HelpFormatter for your purpose without copy-pasting most of _format_action() and using a bunch of private attributes.