Causes sip to generate the type hinting stub (".pyi") files
alongside the generated python modules.

This allows editors like PyCharm to become aware of the
types of objects returned by functions, making the autocompletion
much more useful (e.g. by showing all the members available
for a returned type). It also fixes a bunch of incorrect
warnings highlighted in PyCharm for things like calling
static methods in a class (and, generates some new, but valid,
warnings when unexpected types are passed to functions!)

TODO: work out why PyCharm can't show the docstrings
for methods when the pyi file is present.