Description of problem:

Fails to start - missing libav gstreamer.

[philwyett@ks-binks ~]$ pitivi 
Missing soft dependency:
- libav GStreamer plug-in not found on the system
    -> additional multimedia codecs through the GStreamer Libav library
Traceback (most recent call last):
  File "/usr/lib64/python3.10/site-packages/pitivi/application.py", line 135, in do_startup
    loggable.init('PITIVI_DEBUG', enable_color, enable_crack_output)
  File "/usr/lib64/python3.10/site-packages/pitivi/utils/loggable.py", line 651, in init
    add_limited_log_handler(print_handler)
  File "/usr/lib64/python3.10/site-packages/pitivi/utils/loggable.py", line 738, in add_limited_log_handler
    if not isinstance(func, collections.Callable):
AttributeError: module 'collections' has no attribute 'Callable'
Traceback (most recent call last):
  File "/usr/lib64/python3.10/site-packages/pitivi/application.py", line 203, in do_activate
    self.create_main_window()
  File "/usr/lib64/python3.10/site-packages/pitivi/application.py", line 209, in create_main_window
    self.gui = MainWindow(self)
  File "/usr/lib64/python3.10/site-packages/pitivi/mainwindow.py", line 108, in __init__
    self.app.settings.connect("useDarkThemeChanged",
AttributeError: 'NoneType' object has no attribute 'connect'
[philwyett@ks-binks ~]$

Version-Release number of selected component (if applicable:

2021.05.0-3

How reproducible:

Always.

Steps to Reproduce:
1. Install pitivi.
2. Run from activites icon or in terminal.
3.

Actual results:

Fails to start and crashes.

Expected results:

Start app cleanly.

Additional info: