Map memcached SASL status 0x20 to InvalidCredentials

[jaysonsantos]

Found while writing the SASL integration test for #274.

Evidence

test/test_sasl_integration.py::test_wrong_password_is_rejected connects to a
real memcached started with -S and a Cyrus SASL user database. A wrong
password gives this:

bmemcached.exceptions.MemcachedException: ("Code: 32 Message: b'Auth failure.'", 32)

Status 32 is 0x20. bmemcached/protocol.py maps only 0x08 to
InvalidCredentials:

STATUS = {
    ...
    'auth_error': 0x08,
    ...
}

and in authenticate:

if status == self.STATUS['auth_error']:
    raise InvalidCredentials("Incorrect username or password", status)

So a real authentication failure raises the base MemcachedException, not
InvalidCredentials. A caller that catches InvalidCredentials never sees it.

test/test_auth.py::testAuthUnsuccessful passes today because it mocks
_get_response and feeds 0x08 by hand. No test used a real server, so the
gap was invisible.

Scope

Decide which status codes mean "bad credentials" and map them. 0x20 is
AUTH_ERROR / "Authentication required" in the memcached binary protocol.
Check whether 0x21 (further authentication steps required) needs handling
too.

Acceptance criteria

• A wrong password against a real SASL-enabled memcached raises
  InvalidCredentials.
• test/test_sasl_integration.py::test_wrong_password_is_rejected asserts
  InvalidCredentials in place of MemcachedException.
• test/test_auth.py keeps its mocked coverage of 0x08.

Order

Do #274 first. That issue adds the SASL fixture this work needs.