Disable R0922 in pylint

R0922, “Abstract class is only referenced 1 times”, cannot be disabled
in the source code, and thus
lib/http/auth.py:HttpServerRequestAuthentication gives this warning
(it's actually also referenced from the tests, but we do not include
them).

Therefore we need to disable it at pylintrc level.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Olivier Tharan <olive@google.com>