lint: enable unreachable code detection (#18486)

Author: miketheman

pyproject.toml

@@ -54,6 +54,7 @@ known_first_party = ['warehouse', 'tests']
 
 [tool.mypy]
 python_version = "3.12"
+warn_unreachable = true
 warn_unused_configs = true
 warn_unused_ignores = true
 plugins = ["mypy_zope:plugin"]

warehouse/macaroons/models.py

@@ -106,7 +106,7 @@ def caveats(self, caveats: list[Caveat]):
 
     # Intentionally not using a back references here, since we express
     # relationships in terms of the "other" side of the relationship.
-    user: Mapped["User"] = orm.relationship(lazy=True, viewonly=True)
+    user: Mapped[User | None] = orm.relationship(lazy=True, viewonly=True)
     # TODO: Can't annotate this as "OIDCPublisher" because that would create a
     #  circular import.
     oidc_publisher = orm.relationship("OIDCPublisher", lazy=True, viewonly=True)