Fix test for nightly Python (3.9)

Author: PetterS

pyflakes/test/test_api.py

@@ -449,6 +449,8 @@ def evaluate(source):
         with self.makeTempFile(source) as sourcePath:
             if PYPY:
                 message = 'end of file (EOF) while scanning triple-quoted string literal'
+            elif sys.version_info >= (3, 9):
+                message = 'invalid string prefix'
             else:
                 message = 'invalid syntax'