stripe: Change `assert_called` to `assert_called_once`.
stripe: Change `assert_called` to `assert_called_once`.

Prefer using `assert_called_once` to protect against places where a
mock might be reused, and in so doing have been previously called,
thus making the second usage of `assert_called` not assert anything of
note.

punchagan/zulip,eeshangarg/zulip,zulip/zulip,showell/zulip,showell/zulip,rht/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,showell/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,kou/zulip,punchagan/zulip,rht/zulip,kou/zulip,zulip/zulip,andersk/zulip,andersk/zulip,rht/zulip,punchagan/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,showell/zulip,hackerkid/zulip,rht/zulip,kou/zulip,kou/zulip,kou/zulip,andersk/zulip,andersk/zulip,showell/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip