Add support for other postfix lookup table types

Since Alpine Linux 3.13, Berkeley DB has been deprecated (due to licensing issues). As a lot of containerized software (including Mailu, a well known docker mailserver package) is based on Alpine Linux, I think that mailman should also support other lookup table types, such as lmdb.

Will look at implementing a solution to this in the coming days, even though I'm not that well versed in Python.

p.s. Hope this comes across as an idea/request and not a demand 😄