There are a few stand-alone declarative Postgres tools available, including Stripe's pg-schema-diff [1], Tusker [2] which wraps Migra [3], and sqldef [4] which supports multiple DBMS.
For sake of completeness for users of other databases: for SQLite check out stb-tester's migrator [5], and last but not least for MySQL/MariaDB there's my tool Skeema [6].
[1] https://github.com/stripe/pg-schema-diff
[2] https://github.com/bikeshedder/tusker
[3] https://github.com/djrobstep/migra
[4] https://github.com/sqldef/sqldef/
[5] https://david.rothlis.net/declarative-schema-migration-for-s...