testing/ytmdl: some dependency fixes

• We have py3- prefix as a convention so -python suffix doesn't make sense. This package has just been introduced yesterday and is only used by ytmdl so I don't think we need provides line for backward compatibility.
• Without py3-redis running ytmdl gives me:

$ ytmdl --help
Traceback (most recent call last):
  File "/usr/bin/ytmdl", line 33, in <module>
    sys.exit(load_entry_point('ytmdl==2022.3.16', 'console_scripts', 'ytmdl')())
  File "/usr/bin/ytmdl", line 25, in importlib_load_entry_point
    return next(matches).load()
  File "/usr/lib/python3.10/importlib/metadata/__init__.py", line 171, in load
    module = import_module(match.group('module'))
  File "/usr/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/usr/lib/python3.10/site-packages/ytmdl/__init__.py", line 2, in <module>
    from ytmdl.main import entry
  File "/usr/lib/python3.10/site-packages/ytmdl/main.py", line 34, in <module>
    from ytmdl.core import (
  File "/usr/lib/python3.10/site-packages/ytmdl/core.py", line 7, in <module>
    from ytmdl import (
  File "/usr/lib/python3.10/site-packages/ytmdl/metadata.py", line 11, in <module>
    from ytmdl.meta import (
  File "/usr/lib/python3.10/site-packages/ytmdl/meta/spotify.py", line 8, in <module>
    from spotipy import SpotifyClientCredentials
  File "/usr/lib/python3.10/site-packages/spotipy/__init__.py", line 1, in <module>
    from .cache_handler import *  # noqa
  File "/usr/lib/python3.10/site-packages/spotipy/cache_handler.py", line 14, in <module>
    from redis import RedisError
ModuleNotFoundError: No module named 'redis'

Sorry I should have commented about this in !36783 (merged).