Changelog:

• Allow creating user to delete secure file
• Add a new JUPYTER_PREFER_ENV_PATH variable, which can be set to switch the order of the environment-level path and the user-level path in the Jupyter path hierarchy (e.g., `jupyter --paths`). It is considered set if it is a value that is not one of 'no', 'n', 'off', 'false', '0', or '0.0' (case insensitive). If you are running Jupyter in multiple virtual environments as the same user, you will likely want to set this environment variable.
• Add Python 3.9 builds to testing.
• jupyter --paths --debug now explains the environment variables that affect the current path list.
• Update _version.version_info and _version.__version__ to follow Python conventions.

Packager notes:

• Fix tests per task T9866

Signed-off-by: Algent Albrahimi <algent@protonmail.com>

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