Allow importing of distributions in the Python 3 Lab Controller.

[StykMartin]

Traceback (most recent call last):
  File "<string>", line 5, in <module>
    main()
    ~~~~^^
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 2117, in main
    exit_status.append(build.process(urls, opts))
                       ~~~~~~~~~~~~~^^^^^^^^^^^^
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 920, in process
    full_os_dir = self.get_os_dir()
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 813, in get_os_dir
    os_dir = filter(lambda x: url_exists(x) \
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                    and x, [self.parser.url])[0]
                    ~~~~~~~~~~~~~~~~~~~~~~~~~^^^
TypeError: 'filter' object is not subscriptable

Importing distro from https://archives.fedoraproject.org/pub/archive/fedora/linux/releases/41/Everything/x86_64/os/ ...
Traceback (most recent call last):
  File "<string>", line 5, in <module>
    main()
    ~~~~^^
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 2100, in main
    build = Build(primary_url, options=opts)
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 1950, in Build
    parser = cls.is_importer_for(url, options)
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 1021, in is_importer_for
    if not parser.parse(url):
           ~~~~~~~~~~~~^^^^^
  File "/beaker/LabController/src/bkr/labcontroller/distro_import.py", line 155, in parse
    self.parser.readfp(f)
    ^^^^^^^^^^^^^^^^^^
AttributeError: 'ConfigParser' object has no attribute 'readfp'. Did you mean: 'read'?