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Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

The command line checks for command line utilities are done by trying to
access the executables and by throwing an exception when not found. This
lead to:
- The mat2 cmd line --check-dependencies option failing.
- The ffmpeg unit tests failing when ffmpeg isn't installed (even though
  it's an optional dependency).

This patch fixes it.
Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Invert the keys and values in DEPENDENCIES. It seems more natural to use
the key as a key in check_dependencies(), and the value as the value.
This also help in preparing for reworking the check_dependencies()
helper.
Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Not all id3 types have a text attribute (such as mutagen.id3.APIC or
mutagen.id3.UFID). This leads to the get_meta helper to crash when
trying to access the text attribute of an object which does not have it.
Fixes it by checking the text attribute is available before accessing
it.
Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

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

Additionally, fix a typo.

Closes #96

- More tests
- More documentation
- Minor code cleanup

This should improve epub compatibility,
along with other formats as a side-effect

It's not mandatory to actually have a comment inside
comment delimiter, like `/**/`.

This should close #95.

without /etc/ld.so.cache available in the sandbox, tests fail on gentoo with:
/usr/bin/ffmpeg: error while loading shared libraries: libstdc++.so.6:
    cannot open shared object file: No such file or directory

This should make easier to get more metadata from
archive-based file formats.