Remove backports_abc third-party package (#7533)

The last release was in 2016 and it's not required in any currently
supported Python 3 version.

pyrightconfig.stricter.json

@@ -21,7 +21,6 @@
         "stubs/aws-xray-sdk",
         "stubs/babel",
         "stubs/backports.ssl_match_hostname",
-        "stubs/backports_abc",
         "stubs/bleach",
         "stubs/boto",
         "stubs/beautifulsoup4",