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This switches to use "python3 -m unittest discover -v" onevery distro.

It seems, despite the name, both packages depend on Python 3. However,
pylint3 seems deprecated, and upstream recommends to install pylint:
https://www.pylint.org/#install

The current versions of both packages in Debian unstable are:
pylint  2.4.4-1
pylint3 2.2.2-1

This commit fixes failing CI jobs due to the use of pylint3 2.2.2-1,
which seems broken.

This mounts a new tmpfs on /tmp so any files residing there would be hidden
from the sandbox. Many programs store some files in there that might be useful
to an attacker.  It also drops all capabilities incase it is ever run with
extra capabilities for whatever reason.

- gentoo and debian with bubblewrap are not allowed to fail anymore
- don't run coverage on debian without bubblewrap

It seems, there is a bug somewhere if the test suite is invoked as
'root', and bubblewrap is available.

Closes #93

This made sense some time ago. Nowdays, the whitewhale runner seems to
not really be reliable, so tagging jobs with it doesn't gain much.

Relates #45

Relates #45

Relates #45
Relates #93

This should closes  #90

- This commit introduces optional dependencies (namely ffmpeg):
  mat2 will spit a warning when trying to process an .avi file
  if ffmpeg isn't installed.
- Since metadata are obtained via exiftool, this commit
  also refactors a bit our exfitool wrapper.

Fixes issue #61

This should fix the intermittent failures, thanks
to @pollo for the tip

refactor

- the `src` folder is now `libmat2`
- the `main.py` script is now `mat2.py`