fetch.add_source fails for private ppa on jammy

[verterok]

This is related to lp:2017014 (and maybe lp:1991553)

When adding a private ppa via install_sources from https://github.com/stub42/layer-apt, which ends up calling charmhelpers.fetch.add_source it fails with:

Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 364, in <module>
    sys.exit(0 if addaptrepo.main() else 1)
  File "/usr/bin/add-apt-repository", line 357, in main
    shortcut.add()
  File "/usr/lib/python3/dist-packages/softwareproperties/shortcuthandler.py", line 222, in add
    self.add_key()
  File "/usr/lib/python3/dist-packages/softwareproperties/shortcuthandler.py", line 398, in add_key
    if not all((self.trustedparts_file, self.trustedparts_content)):
  File "/usr/lib/python3/dist-packages/softwareproperties/ppa.py", line 141, in trustedparts_content
    key = self.lpppa.getSigningKeyData()
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/resource.py", line 592, in __call__
    response, content = self.root._browser._request(
  File "/usr/lib/python3/dist-packages/lazr/restfulclient/_browser.py", line 429, in _request
    raise error
lazr.restfulclient.errors.Unauthorized: HTTP Error 401: Unauthorized
Response headers:
---
-content-encoding: gzip
content-length: 78
content-security-policy: frame-ancestors 'self';
content-type: text/plain;charset=utf-8
date: Wed, 19 Apr 2023 13:36:06 GMT
server: gunicorn
status: 401
strict-transport-security: max-age=15552000
vary: Accept,Accept-Encoding
x-content-type-options: nosniff
x-frame-options: SAMEORIGIN
x-launchpad-revision: 473868c1cc6b58a9bc722c23840374c93a7a274b
x-lazr-notifications: []
x-powered-by: Zope (www.zope.org), Python (www.python.org)
x-request-id: 7744610e-481a-4489-ad5b-e9288c8b7588
x-vcs-revision: 473868c1cc6b58a9bc722c23840374c93a7a274b
x-xss-protection: 1; mode=block
---

This is because of changes to add-apt-repository, which now treats evething as a PPA and tries to get information from launchpad via API calls. if the ppa is private, such attemps will fail and here we are now :-)