Also fixes a bug in process_initial_upgrade. If you have a card on file
(e.g. from a previous subscription), and try to upgrade by billing by
invoice, neither the if nor the elif condition applies.
There is no attribute called plan_interval. The reason
this was missed by mypy is moost likely due to us using
Any instead of Subscription in extract_current_subscription.
This will make it easier to mock the calls in our new stripe mocking
framework. I believe the two forms are equivalent, assuming the Stripe
Python bindings aren't doing anything crazy. And if not, well hopefully our
new testing framework will catch it :).
[Idea originally from Vishnu KS.]