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

The upgrade script for auto DTMF mode (31cd4f4891ec) added in 88b0fa7755
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

contrib/ast-db-manage/config/versions/31cd4f4891ec_add_auto_dtmf_mode.py

@@ -11,6 +11,7 @@ revision = '31cd4f4891ec'
 down_revision = '23530d604b96'
 
 from alembic import op
+from sqlalchemy.dialects.postgresql import ENUM
 import sqlalchemy as sa
 
 OLD_ENUM = ['rfc4733', 'inband', 'info']