fix[tool]: update InterfaceT.__str__ implementation (#4205)

this doesn't affect anything in vyper codebase, but affects the
serialization of interface types for tooling (including titanoboa)

tests/functional/syntax/test_external_calls.py

@@ -305,7 +305,7 @@ def bar():
     extcall Foo(msg.sender)
     """,
         StructureException,
-        "Function `type(interface Foo)` cannot be called without assigning the result",
+        "Function `type(Foo)` cannot be called without assigning the result",
         None,
     ),
 ]

vyper/semantics/types/module.py

@@ -76,6 +76,9 @@ def getter_signature(self):
     def abi_type(self) -> ABIType:
         return ABI_Address()
 
+    def __str__(self):
+        return self._id
+
     def __repr__(self):
         return f"interface {self._id}"