Django-pg-zero-downtime-migrations

- added django 3.1 support
- added postgres 13 support
- drop python 3.5 support
- updated test environment

- fixed decimal to float migration error
- fixed django 3.0.2+ tests

- added django 3.0 support
- added concurrently index creation and removal operations
- added exclude constraint support as unsafe operation
- drop postgres 9.4 support
- drop django 2.0 support
- drop django 2.1 support
- drop deprecated `django_zero_downtime_migrations_postgres_backend` module

- added python 3.8 support
- added postgres specific indexes support
- improved tests clearness
- fixed regexp escaping warning for management command
- fixed style check
- improved README
- marked python 3.5 support deprecated
- marked postgres 9.4 support deprecated
- marked django 2.0 support deprecated
- marked django 2.1 support deprecated

- marked `ZERO_DOWNTIME_MIGRATIONS_USE_NOT_NULL` option deprecated for postgres 12+
- added management command for migration to real `NOT NULL` from `CHECK IS NOT NULL` constraint
- added integration tests for pg 12, pg 11 root, pg 11 compatible not null constraint, pg 11 standard not null constraint and pg 10, 9.6, 9.5, 9.4, postgis databases
- fixed compatible check not null constraint deletion and creation via pg_attribute bugs
- minimized side affect with deferred sql execution between operations in one migration module
- added postgres 12 safe `NOT NULL` constraint creation
- added safe `NOT NULL` constraint creation for extra permissions for `pg_catalog.pg_attribute` with `ZERO_DOWNTIME_MIGRATIONS_USE_NOT_NULL=USE_PG_ATTRIBUTE_UPDATE_FOR_SUPERUSER` option enabled
- marked `AddField` with `null=False` parameter and compatible `CHECK IS NOT NULL` constraint option as unsafe operation and avoid `ZERO_DOWNTIME_MIGRATIONS_USE_NOT_NULL` value in this case
- added version to package
- fixed pypi README images links
- improved README

- extracted zero-downtime-schema to mixin to allow use this logic with other backends
- moved module from `django_zero_downtime_migrations_postgres_backend` to `django_zero_downtime_migrations.backends.postgres`
- marked `django_zero_downtime_migrations_postgres_backend` module as deprecated
- added postgis backend support
- improved README