Let _core.get_ffmpeg_library_versions() return a dict of namedtuples instead of a dict of strings

[NicolasHug]

_core.get_ffmpeg_library_versions() returns a dict of strings. As discussed in D58974580, versions are better exposed as namedtuples, like so:

[nav] In [25]: _Version = namedtuple("_Version", ("major", "minor", "micro"))
[ins] In [26]: version = _Version(1, 2, 3)
[ins] In [27]: version
Out[27]: _Version(major=1, minor=2, micro=3)
[ins] In [28]: version < (1, 2, 4)
Out[28]: True
[ins] In [29]: version < (1, 2, 3, 4)
Out[29]: True
[ins] In [30]: version.major
Out[30]: 1

This is the same kind of struct that sys.version_info returns (we can't use their base class, but that doesn't matter).