python3-stdeb depends on python-all which no longer exists on focal

[Marcus Furlong]

$ ./setup.py --command-packages=stdeb.command bdist_deb
running bdist_deb
running sdist_dsc
running egg_info

[...]

dpkg-query: no packages found matching python-all
ERROR running: /usr/bin/dpkg-query --show --showformat=${Version} python-all
Traceback (most recent call last):
  File "./setup.py", line 32, in <module>
    setup(
  File "/usr/lib/python3/dist-packages/setuptools/__init__.py", line 144, in setup
    return distutils.core.setup(**attrs)
  File "/usr/lib/python3.8/distutils/core.py", line 148, in setup
    dist.run_commands()
  File "/usr/lib/python3.8/distutils/dist.py", line 966, in run_commands
    self.run_command(cmd)
  File "/usr/lib/python3.8/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/usr/lib/python3/dist-packages/stdeb/command/bdist_deb.py", line 22, in run
    self.run_command('sdist_dsc')
  File "/usr/lib/python3.8/distutils/cmd.py", line 313, in run_command
    self.distribution.run_command(command)
  File "/usr/lib/python3.8/distutils/dist.py", line 985, in run_command
    cmd_obj.run()
  File "/usr/lib/python3/dist-packages/stdeb/command/sdist_dsc.py", line 135, in run
    build_dsc(debinfo,
  File "/usr/lib/python3/dist-packages/stdeb/util.py", line 1369, in build_dsc
    python_defaults_version_str = get_version_str('python-all')
  File "/usr/lib/python3/dist-packages/stdeb/util.py", line 268, in get_version_str
    stdout = get_cmd_stdout(args)
  File "/usr/lib/python3/dist-packages/stdeb/util.py", line 243, in get_cmd_stdout
    raise RuntimeError('returncode %d', returncode)
RuntimeError: ('returncode %d', 1)

To fix it, just need to change python-all to python3-all:

$ sudo sed -i -e "s/python-all/python3-all/g" /usr/lib/python3/dist-packages/stdeb/util.py