Consider adding support for Ubuntu Focal

[eloquence]

Ubuntu 20.04 is still supported until April 2025, so we should consider adding it to the list of supported releases for users who haven't moved to Jammy yet.

[deeplow]

Apparently podman (on which the linux version depdends) is not available on ubuntu focal and it requires external repos (linked from the podman blog).

And actually we do test podman on this exact ubuntu version (it's where the CI tests the document conversion process). And dangerzone had been available in the past for that ubuntu version.

@apyrgio will add the documentation to the installation instructions after successful testing & confirmation the packagecloud deb works for that version.

[deeplow]

It doesn't support python3.10 (which we now need) unless we add the deadsnakes as a dependency

[apyrgio]

Our Packagecloud repos for Ubuntu Focal do have a Dangerzone package, but the latest version is 0.2.1, which is the last version that supported Ubuntu Focal. See also #160 (comment). This means that we need to partially revert 229ebbd, and create a 0.4 Ubuntu Focal package, besides adding Podman instructions.

Aside from that, I'd like to ask a quick question:

It doesn't support python3.10 (which we now need) unless we add the deadsnakes as a dependency

I believe this is no longer accurate, right? We had some minor Mypy issues in Python 3.9, but they are not important enough to switch to Python 3.10.

[apyrgio]

I can verify that we can install Dangerzone on Ubuntu Focal, if we install Podman first, with the instructions that we have in our CircleCI config. As for the .deb itself, I just downloaded it from Packagecloud, without adding the repo in the sources.

I haven't managed to test Dangerzone successfully though, because I encountered this issue:

/usr/lib/python3/dist-packages/click/core.py:940: Warning: Invoked legacy parameter callback "<function validate_input_filename at 0x7fc0e8cb61f0>".  The new signature for such callbacks starting with click 2.0 is (ctx, param, value).
  value, args = param.handle_parse_result(ctx, opts, args)
Traceback (most recent call last):
  File "/usr/bin/dangerzone", line 33, in <module>
    sys.exit(load_entry_point('dangerzone==0.3.2', 'console_scripts', 'dangerzone')())
  File "/usr/lib/python3/dist-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/click/core.py", line 716, in main
    with self.make_context(prog_name, args, **extra) as ctx:
  File "/usr/lib/python3/dist-packages/click/core.py", line 641, in make_context
    self.parse_args(ctx, args)
  File "/usr/lib/python3/dist-packages/click/core.py", line 940, in parse_args
    value, args = param.handle_parse_result(ctx, opts, args)
  File "/usr/lib/python3/dist-packages/click/core.py", line 1476, in handle_parse_result
    value = invoke_param_callback(
  File "/usr/lib/python3/dist-packages/click/core.py", line 95, in invoke_param_callback
    return callback(ctx, value)
  File "/usr/lib/python3/dist-packages/dangerzone/errors.py", line 23, in wrapper
    return func(*args, **kwargs)
TypeError: validate_input_filename() missing 1 required positional argument: 'value'

Turns out that Click 7.x and below checked for the arguments that callbacks support, and if they were less than 3, it invoked the callback with less arguments (legacy mode):

https://github.com/pallets/click/blob/1784558ed7c75c65764d2a434bd9cbb206ca939d/src/click/core.py#L108-L112

In our case, our callback does support three arguments, but it's decorated with another function. This shouldn't be a problem because we use functools.wraps(func), but turns out that it doesn't suffice, and the reported arguments for the function are 0. Others have encountered a similar problem, so we need to solve it similarly.