fix stevedore.exception.NoMatches: No jjb.cli.subcommands extensions found

[maorfr]

sh-4.4# jenkins-jobs --version
Traceback (most recent call last):
  File "/usr/local/bin/jenkins-jobs", line 33, in <module>
    sys.exit(load_entry_point('jenkins-job-builder==2.10.1', 'console_scripts', 'jenkins-jobs')())
  File "/usr/local/lib/python3.6/site-packages/jenkins_job_builder-2.10.1-py3.6.egg/jenkins_jobs/cli/entry.py", line 164, in main
    jjb = JenkinsJobs(argv)
  File "/usr/local/lib/python3.6/site-packages/jenkins_job_builder-2.10.1-py3.6.egg/jenkins_jobs/cli/entry.py", line 58, in __init__
    self.parser = create_parser()
  File "/usr/local/lib/python3.6/site-packages/jenkins_job_builder-2.10.1-py3.6.egg/jenkins_jobs/cli/parser.py", line 100, in create_parser
    extension_manager.map(parse_subcommand_args, subparser)
  File "/usr/local/lib/python3.6/site-packages/stevedore-3.3.0-py3.6.egg/stevedore/extension.py", line 291, in map
    raise NoMatches('No %s extensions found' % self.namespace)
stevedore.exception.NoMatches: No jjb.cli.subcommands extensions found

slack thread: https://coreos.slack.com/archives/GGC2A0MS8/p1622140434388400

[maorfr]

reported in python/importlib_metadata#320