Django-pg-zero-downtime-migrations

- marked the `ZERO_DOWNTIME_MIGRATIONS_USE_NOT_NULL` option as deprecated for postgres 12+
- added a management command for migrating from a `CHECK IS NOT NULL` constraint to a real `NOT NULL` constraint
- added integration tests for postgres 12, postgres 11 (root), postgres 11 with compatible not null constraints, postgres 11 with standard not null constraints, as well as postgres 10, 9.6, 9.5, 9.4, and postgis databases
- fixed bugs related to the deletion and creation of compatible check not null constraints via `pg_attribute`
- minimized side effects with deferred sql execution between operations in one migration module
- added safe `NOT NULL` constraint creation for postgres 12
- added safe `NOT NULL` constraint creation for extra permissions for `pg_catalog.pg_attribute` when the `ZERO_DOWNTIME_MIGRATIONS_USE_NOT_NULL=USE_PG_ATTRIBUTE_UPDATE_FOR_SUPERUSER` option is enabled
- marked `AddField` with the `null=False` parameter and the compatible `CHECK IS NOT NULL` constraint option as an unsafe operation, ignoring the `ZERO_DOWNTIME_MIGRATIONS_USE_NOT_NULL` value in this case
- added versioning to the package
- fixed pypi README image links
- improved README

- extracted zero-downtime-schema logic into a mixin to allow using it with other backends
- moved the module from `django_zero_downtime_migrations_postgres_backend` to `django_zero_downtime_migrations.backends.postgres`
- marked the `django_zero_downtime_migrations_postgres_backend` module as deprecated
- added support for the postgis backend
- improved README

- changed the defaults for `ZERO_DOWNTIME_MIGRATIONS_LOCK_TIMEOUT` and `ZERO_DOWNTIME_MIGRATIONS_STATEMENT_TIMEOUT` from `0ms` to `None` to match the default django behavior that respects postgres timeouts
- updated the documentation with option defaults
- updated the documentation with best practices for option usage
- fixed the issue where adding a nullable field with a default did not raise an error or warning
- added links to the documentation describing the issue and safe alternative usage for errors and warnings
- updated the documentation with type casting workarounds

- added django 2.2 support with the `Meta.indexes` and `Meta.constraints` attributes
- fixed python deprecation warnings for regular expressions
- removed the unused `TimeoutException`
- improved README and PYPI description

- added an option to disable `statement_timeout` for long operations, such as index creation and constraint validation, when `statement_timeout` is set globally

- added long description content type