python 3.8 support

• 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

postgres 12 support and NOT NULL constraint improvements

• 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

postgis support

• 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