fixtures: make the FixtureValue TypeVar covariant

This TypeVar represents the return value of the fixture function, so
should be covariant.

Author: bluetech

src/_pytest/fixtures.py

@@ -82,7 +82,7 @@
 
 
 # The value of the fixture -- return/yield of the fixture function (type variable).
-FixtureValue = TypeVar("FixtureValue")
+FixtureValue = TypeVar("FixtureValue", covariant=True)
 # The type of the fixture function (type variable).
 FixtureFunction = TypeVar("FixtureFunction", bound=Callable[..., object])
 # The type of a fixture function (type alias generic in fixture value).