In the OpenType 1.8 specification, this is called TupleVariation
so let's be consistent with the spec. (The initial implementation
in fonttools pre-dates OpenType 1.8).
Here I use logging.basicConfig to configure the root logger, and hence all the other loggers,
including fontTools'.
I could have used fontTools.configLogger as well (and get predefined formatter, etc.), but wanted
to show that one can also configure the fontTools logger using the built-in logging configuration
functions (e.g. basicConfig, config.dictConfig and config.fileConfig).
With this change, the "Thin" instance now renders fine, too.
Apparently, the axis values need to be positive; this was not
clear to me from reading the specification.
