Google App Engine Standard Environment Services SDK for Python 3

The `appengine-python-standard` library is the Google App Engine services SDK for Python 3. It provides Python 3 applications running in the App Engine standard environment access to various legacy bundled services and API endpoints that were previously only available on the Python 2.7 runtime, such as Mail, Memcache, NDB, and URL Fetch. The library is actively maintained, with minor, patch, and occasional major releases addressing new features and compatibility.