what to do with DBContainer - split from removing sqlalchemy dependency

[alexanderankin]

from https://github.com/testcontainers/testcontainers-python/pull/445#issuecomment-1985436334

After this discussion (with adding _wait_for_ready() and _configure() to the top level class), DBContainer would only contain this: ... Anyway: I would appreciate if this conversation could be moved to a separate issue :-)