Relates #106

As such, hopefully, it's not really used widely anymore. If so, this
note isn't really relevant.

Also, make the note generic, to omit the need to update it "constantly".

Closes #76

This shouldn't make a big difference in the CLI/extension
usage, but might improve the performances of long-running
instances, or people misusing the API.

Rework the dependencies definition to include a 'required' flags, which
is passed by the check_dependencies helper to the callers, so that they
can distinguish between required and optional dependencies.

This help in two ways:
- The unit test for the dependencies was now failing when an optional
  one was missing, due to a previous rework.
- Mat2's --check-dependencies was referring to "required dependencies"
  and was misleading for the user as some of them could be optional.
Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

Remove the try/except logic when calling check_dependencies, as it
cannot throw the exception anymore (it's caught already in the
function).
Signed-off-by: Antoine Tenart <antoine.tenart@ack.tf>

This should ensure that if we decide to implement
some threading in the cli, a faulty file
won't break everything.

This one was spotted by @fuzzy

Apparently, mypy isn't able (yet?) to deal
with variables that are changing their types
at runtime.

Python is wonderful.

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>