Add stronger tests for signing.verify
Add stronger tests for signing.verify

- on python 2 test that both correct `unicode` and `str` signatures
  are handled which is a behavior we support ATM
- test that the function won't crash on a wrong unicode input
  (it does not ATM)

The motivation is that if we were to switch to a different constant
time compare function (which may result in these tests failing),
we want to be backwards compatible.

laterpay/laterpay-client-python