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

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.

The issue identifies no files, tests, or entry points. Start by locating the project’s Python-version compatibility checks and test configuration, then determine which maintained-version changes are required. Done means standalone gsutil no longer promises or tests support for Python 3.5, 3.6, or 3.7 while Python 3.8 and later remain supported.

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