PY-73246 Don't insert square brackets after classes not having free type parameters

Even if they transitively extend typing.Generic, such as the builtin str.

GitOrigin-RevId: 9389ef5846e21ba5e0fed8b835beb2d458e42f13

python/testData/completion/squareBracketsNotInsertedAfterAlreadyParameterizedGenericInsideTypeHints/a.after.py

@@ -0,0 +1,8 @@
+from typing import Iterable
+
+
+class StrIterable(Iterable[str]):
+    pass
+
+
+x: StrIterable

python/testData/completion/squareBracketsNotInsertedAfterAlreadyParameterizedGenericInsideTypeHints/a.py

@@ -0,0 +1,8 @@
+from typing import Iterable
+
+
+class StrIterable(Iterable[str]):
+    pass
+
+
+x: StrIter<caret>