Vapid01 instance is not accepted as a key object

[mthu]

Since Vapid01 is a superclass of Vapid02 (and thus Vapid) in py_vapid library, following condition does not match the Vapid01 instance and the ongoing code treats the object as a string (and fails parsing it).

pywebpush/pywebpush/__init__.py

Line 461 in a2dbc6f

if isinstance(vapid_private_key, Vapid):