fixtures: only call instance property once in function

No need to compute the property multiple times.

Author: bluetech

src/_pytest/fixtures.py

@@ -1101,17 +1101,18 @@ def resolve_fixture_function(
     # The fixture function needs to be bound to the actual
     # request.instance so that code working with "fixturedef" behaves
     # as expected.
-    if request.instance is not None:
+    instance = request.instance
+    if instance is not None:
         # Handle the case where fixture is defined not in a test class, but some other class
         # (for example a plugin class with a fixture), see #2270.
         if hasattr(fixturefunc, "__self__") and not isinstance(
-            request.instance,
+            instance,
             fixturefunc.__self__.__class__,  # type: ignore[union-attr]
         ):
             return fixturefunc
         fixturefunc = getimfunc(fixturedef.func)
         if fixturefunc != fixturedef.func:
-            fixturefunc = fixturefunc.__get__(request.instance)  # type: ignore[union-attr]
+            fixturefunc = fixturefunc.__get__(instance)  # type: ignore[union-attr]
     return fixturefunc