[Bug 1943638] [NEW] apt_check.py crashes with exception "AttributeError"

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Public bug reported:

On my Ubuntu server, I received the message for quite some time (> 1 year): 
"The list of available updates is more than a week old"

I regularly run "apt update", so the message simply isn't true.
After some research, I manage to identify the script "/usr/lib/update-notifier/apt-check" as the root cause.
When executed with parameter "--human-readable", the following exception is thrown.
Without "--human-readable", no exception is thrown.

Traceback (most recent call last):
  File "/usr/lib/python3.8/pdb.py", line 1705, in main
    pdb._runscript(mainpyfile)
  File "/usr/lib/python3.8/pdb.py", line 1573, in _runscript
    self.run(statement)
  File "/usr/lib/python3.8/bdb.py", line 580, in run
    exec(cmd, globals, locals)
  File "<string>", line 1, in <module>
  File "/usr/lib/update-notifier/apt_check.py", line 6, in <module>
    import apt
  File "/usr/lib/update-notifier/apt_check.py", line 477, in run
    write_human_readable_summary(sys.stdout, upgrades, security_updates,
  File "/usr/lib/update-notifier/apt_check.py", line 249, in write_human_readable_summary
    esm_distro = is_esm_distro()
  File "/usr/lib/update-notifier/apt_check.py", line 150, in is_esm_distro
    DISTRO in ubuntu_distro.supported_esm()
AttributeError: 'UbuntuDistroInfo' object has no attribute 'supported_esm'
Uncaught exception. Entering post mortem debugging


Ubuntu version: 

# cat /etc/os-release
NAME="Ubuntu"
VERSION="20.04.3 LTS (Focal Fossa)"
ID=ubuntu
ID_LIKE=debian
PRETTY_NAME="Ubuntu 20.04.3 LTS"
VERSION_ID="20.04"
HOME_URL="https://www.ubuntu.com/"
SUPPORT_URL="https://help.ubuntu.com/"
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal

** Affects: update-notifier (Ubuntu)
     Importance: Undecided
         Status: New

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https://bugs.launchpad.net/bugs/1943638

Title:
  apt_check.py crashes with exception "AttributeError"

Status in update-notifier package in Ubuntu:
  New

Bug description:
  On my Ubuntu server, I received the message for quite some time (> 1 year): 
  "The list of available updates is more than a week old"

  I regularly run "apt update", so the message simply isn't true.
  After some research, I manage to identify the script "/usr/lib/update-notifier/apt-check" as the root cause.
  When executed with parameter "--human-readable", the following exception is thrown.
  Without "--human-readable", no exception is thrown.

  Traceback (most recent call last):
    File "/usr/lib/python3.8/pdb.py", line 1705, in main
      pdb._runscript(mainpyfile)
    File "/usr/lib/python3.8/pdb.py", line 1573, in _runscript
      self.run(statement)
    File "/usr/lib/python3.8/bdb.py", line 580, in run
      exec(cmd, globals, locals)
    File "<string>", line 1, in <module>
    File "/usr/lib/update-notifier/apt_check.py", line 6, in <module>
      import apt
    File "/usr/lib/update-notifier/apt_check.py", line 477, in run
      write_human_readable_summary(sys.stdout, upgrades, security_updates,
    File "/usr/lib/update-notifier/apt_check.py", line 249, in write_human_readable_summary
      esm_distro = is_esm_distro()
    File "/usr/lib/update-notifier/apt_check.py", line 150, in is_esm_distro
      DISTRO in ubuntu_distro.supported_esm()
  AttributeError: 'UbuntuDistroInfo' object has no attribute 'supported_esm'
  Uncaught exception. Entering post mortem debugging

  
  Ubuntu version: 

  # cat /etc/os-release
  NAME="Ubuntu"
  VERSION="20.04.3 LTS (Focal Fossa)"
  ID=ubuntu
  ID_LIKE=debian
  PRETTY_NAME="Ubuntu 20.04.3 LTS"
  VERSION_ID="20.04"
  HOME_URL="https://www.ubuntu.com/"
  SUPPORT_URL="https://help.ubuntu.com/"
  BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/"
  PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy"
  VERSION_CODENAME=focal
  UBUNTU_CODENAME=focal

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