contrib/ast-db-manage: Add Postgres ENUM type support in auto DTMF mode update

The upgrade script for auto DTMF mode (31cd4f4891ec) added in 88b0fa77
failed to add ENUM support for Postgres databases. This requires a
specific import from the sqlalchemy.dialects.postgresql package. This
patch corrects this error, which allows for Postgres update scripts to
be generated.

ASTERISK-24706

Change-Id: I4742ac8efa533cd6f18e0bdd907b339a9aedf015