ZKAPAuthorizer depends on "privacypass" which no longer exists

Created by: crwood

As per https://github.com/LeastAuthority/python-challenge-bypass-ristretto/pull/24, the "privacypass" package has been renamed to "python-challenge-bypass-ristretto" (or "challenge-bypass-ristretto"? or "challenge_bypass_ristretto"?), however, ZKAPAuthorizer continues to depend upon and search for the "privacypass" namespace in numerous places.