DatabaseClient: add missing class methods and reflect Django 3.2 refactor (#846)

* DatabaseClient: add missing class method `settings_to_cmd_args_env` and add missing arg to `runshell`

The `DatabaseClient.runshell()` was refactored in Django 3.2 to be more standardized
across the different database backends.

* DatabaseClient: make `settings_dict` dict type better by specifying key/value types

* tests: add missing db backend client files to `import_all_modules`

tests/typecheck/test_import_all.yml

@@ -231,6 +231,7 @@
         import django.db.backends.dummy.base
         import django.db.backends.mysql
         import django.db.backends.mysql.client
+        import django.db.backends.oracle.client
         import django.db.backends.postgresql
         import django.db.backends.postgresql.base
         import django.db.backends.postgresql.client
@@ -239,6 +240,7 @@
         import django.db.backends.signals
         import django.db.backends.sqlite3
         import django.db.backends.sqlite3.base
+        import django.db.backends.sqlite3.client
         import django.db.backends.sqlite3.creation
         import django.db.backends.sqlite3.features
         import django.db.backends.sqlite3.introspection