standardize mod name
The PEP says that the module name is "sentinels" (plural) but the example imports Sentinel from the "sentinel" (singular) module. Not absolutely sure which one is intended but recent standards (secrets, dataclasses) have used the plural.
CLA for Python signed, in case that's needed.
Co-authored-by: Hugo van Kemenade <hugovk@users.noreply.github.com>