Add unbound typevar case

Not exactly a semanal error, but could be and adds useful context

Author: brianschubert

test-data/unit/semanal-errors.test

@@ -1072,6 +1072,9 @@ class C(Generic[T]):
     Bad = TypeVar("Bad", int, List[T])    # E: TypeVar constraint type cannot be parametrized by type variables
 class D:
     Bad = TypeVar("Bad", int, List[Self]) # E: TypeVar constraint type cannot be parametrized by type variables
+S = TypeVar("S", int, List[T])            # E: Type variable "__main__.T" is unbound \
+                                          # N: (Hint: Use "Generic[T]" or "Protocol[T]" base class to bind "T" inside a class) \
+                                          # N: (Hint: Use "T" in function signature to bind "T" inside a function)
 
 [case testInvalidTypevarValues]
 from typing import TypeVar