ref: enable optional redundant-self mypy code (#68718)

none of our code currently fails this

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pyproject.toml

@@ -64,7 +64,7 @@ warn_unreachable = true
 warn_unused_configs = true
 warn_unused_ignores = true
 warn_redundant_casts = true
-enable_error_code = ["ignore-without-code"]
+enable_error_code = ["ignore-without-code", "redundant-self"]
 
 [tool.django-stubs]
 django_settings_module = "sentry.conf.server_mypy"