Incorrect symlink checking => Your python3 install is corrupted.

This is related to https://bugs.launchpad.net/bugs/1825655.

When trying to upgrade to jammy, I got
```
Your python3 install is corrupted. Please fix the '/usr/bin/python3'
symlink.
```

Digging into the matter, it comes down to this symlink checker in `DistUpgradeController.py`:
```
import logging
from configparser import ConfigParser
import os
from configparser import NoOptionError

logging.basicConfig(filename='example.log', encoding='utf-8', level=logging.DEBUG)

def _pythonSymlinkCheck():
    """ sanity check that /usr/bin/python3 points to the default
        python version. Users tend to modify this symlink, which
        breaks stuff in obscure ways (Ubuntu #75557).
    """
    logging.debug("_pythonSymlinkCheck run")
    binaries_and_dirnames = [("python3", "python3")]
    for binary, dirname in binaries_and_dirnames:
        debian_defaults = '/usr/share/%s/debian_defaults' % dirname
        if os.path.exists(debian_defaults):
            config = ConfigParser()
            with open(debian_defaults) as f:
                config.read_file(f)
            try:
                expected_default = config.get('DEFAULT', 'default-version')
            except NoOptionError:
                logging.debug("no default version for %s found in '%s'" %
                              (binary, config))
                return False
            try:
                fs_default_version = os.readlink('/usr/bin/%s' % binary)
            except OSError as e:
                logging.error("os.readlink failed (%s)" % e)
                return False
            if not fs_default_version in (expected_default, os.path.join('/usr/bin', expected_default)):
                logging.debug("%s symlink points to: '%s', but expected is '%s' or '%s'" %
                              (binary, fs_default_version, expected_default, os.path.join('/usr/bin', expected_default)))
                return False
    return True

res = _pythonSymlinkCheck()
print(res)
```
Running it indeed returns `False` on my machine. The log gives the reason:
```
DEBUG:root:_pythonSymlinkCheck run
DEBUG:root:python3 symlink points to: '/etc/alternatives/python3', but expected is 'python3.9' or '/usr/bin/python3.9'
```
The symlink is not followed through.

Instead of `os.readlink`, you want to use `os.path.realpath`.