Dropping Official Support for Python 3.5, 3.6, and 3.7 in gsutil

[sameer-google]

We would like to inform you that, starting with the upcoming releases, gsutil will no longer support Python versions 3.5, 3.6, and 3.7. This decision is based on several factors, including the end of life for these Python versions and the need to streamline our support for newer versions that provide better performance, security, and compatibility with modern libraries.

Please note that gsutil bundled with the Google Cloud SDK has already dropped support for Python 3.5, 3.6, and 3.7. While standalone gsutil may still function with these older Python versions, we will no longer be testing or maintaining compatibility with them moving forward. We highly recommend updating to Python 3.8 or later for continued support and an improved experience.