Typing stubs for backports

The `types-backports` library provides automatically generated PEP 561 type stub packages for various 'backports' libraries. These stubs enable static type checkers like mypy, PyCharm, and pytype to validate code that utilizes features backported to older Python versions. The package is generated from the Python typeshed repository. The primary package itself is largely a meta-package, with more specific stubs often residing in `types-backports.*` packages. The current version is 0.1.3, released in June 2021, and its active development for general backports has largely ceased.