[Bug 2015243] [NEW] py3versions doesn't follow symlinks (like realpath)

[Kostiantyn Astakhov]

Public bug reported:

In case if we have more than one python version and manage them by update-alternatives, the path to the python binary looks like:
root at devvm-14-143:~# ls -la /usr/bin/python3
lrwxrwxrwx 1 root root 25 Apr  4 08:11 /usr/bin/python3 -> /etc/alternatives/python3
root at devvm-14-143:~# ls -la /etc/alternatives/python3
lrwxrwxrwx 1 root root 18 Apr  4 08:11 /etc/alternatives/python3 -> /usr/bin/python3.8

So even we use the correct default version of python3 (like in Focal it
is python3.8) py3version will fail

root at devvm-14-143:~# py3versions -d
py3versions: the symlink /usr/bin/python3 does not point to the python3 default version. It must be reset to point to python3.8

This caused due to os.readlink doesn't follow multiply symlinks, so I
suggest to use Path from pathlib.

I have attached patch with suggested changes.

** Affects: python3-defaults (Ubuntu)
     Importance: Undecided
         Status: New

** Patch added: "py3versions.patch"
   https://bugs.launchpad.net/bugs/2015243/+attachment/5660888/+files/py3versions.patch

** Summary changed:

- py3versions desn't follow symlinks (like realpath)
+ py3versions doesn't follow symlinks (like realpath)

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https://bugs.launchpad.net/bugs/2015243

Title:
  py3versions doesn't follow symlinks (like realpath)

Status in python3-defaults package in Ubuntu:
  New

Bug description:
  In case if we have more than one python version and manage them by update-alternatives, the path to the python binary looks like:
  root at devvm-14-143:~# ls -la /usr/bin/python3
  lrwxrwxrwx 1 root root 25 Apr  4 08:11 /usr/bin/python3 -> /etc/alternatives/python3
  root at devvm-14-143:~# ls -la /etc/alternatives/python3
  lrwxrwxrwx 1 root root 18 Apr  4 08:11 /etc/alternatives/python3 -> /usr/bin/python3.8

  So even we use the correct default version of python3 (like in Focal
  it is python3.8) py3version will fail

  root at devvm-14-143:~# py3versions -d
  py3versions: the symlink /usr/bin/python3 does not point to the python3 default version. It must be reset to point to python3.8

  This caused due to os.readlink doesn't follow multiply symlinks, so I
  suggest to use Path from pathlib.

  I have attached patch with suggested changes.

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