There are several situations in which we want to create a Customer and
stripe.Customer object before we really have a billing relationship with a
customer. The main one is giving non-profit or educational discounts.
The main purpose of this commit is to demonstrate end-to-end that
our setup for type stubs works. I picked this library more or less
arbitrarily as one where the API surface we use is tiny, so that the
stub would be extra easy to write.