Calibre crashes on Ubuntu 20.04

[Heiko Finzel]

After some Package Updates Calibre stopped working.
Error is:

```
~$ calibre
Traceback (most recent call last):
  File "/usr/bin/calibre", line 20, in <module>
    sys.exit(calibre())
  File "/usr/lib/calibre/calibre/gui_launch.py", line 73, in calibre
    main(args)
  File "/usr/lib/calibre/calibre/gui2/main.py", line 543, in main
    listener = create_listener()
  File "/usr/lib/calibre/calibre/gui2/main.py", line 514, in create_listener
    return Listener(address=gui_socket_address())
  File "/usr/lib/calibre/calibre/utils/ipc/server.py", line 110, in __init__
    self._listener._unlink.cancel()
AttributeError: 'NoneType' object has no attribute 'cancel'
```
So it seems it has something to do with "gui socket address".

I can't tell exactly when it broke since I don't use it daily but it definitly worked on my Ubuntu 20.04 last time I used it.

Versions/Installation:
* calibre (calibre 4.99.4)
* python: 3.8.5
* kernel version (uname): Linux hfi-ux433fn 5.4.0-56-generic #62-Ubuntu SMP Mon Nov 23 19:20:19 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux
* I have nothing really special at my installation & calibre comes from Ubuntu Repos
* Packages of the whole system have the update state of right now (3.12.2020)

[Kovid Goyal (kovid)]

The calibre package in Ubuntu is broken, please uninstall it and use the official calibre binaries from https://calibre-ebook.com/download_linux
See https://lists.ubuntu.com/archives/ubuntu-devel/2020-October/041228.html

 duplicate 1898904