use assert[Not]In where appropriate

A patch from Dave Malcolm.
diff --git a/Lib/test/test_compile.py b/Lib/test/test_compile.py
index 563a7ee..af4572f 100644
--- a/Lib/test/test_compile.py
+++ b/Lib/test/test_compile.py
@@ -389,10 +389,10 @@
                 import __mangled_mod
                 import __package__.module
 
-        self.assertTrue("_A__mangled" in A.f.__code__.co_varnames)
-        self.assertTrue("__not_mangled__" in A.f.__code__.co_varnames)
-        self.assertTrue("_A__mangled_mod" in A.f.__code__.co_varnames)
-        self.assertTrue("__package__" in A.f.__code__.co_varnames)
+        self.assertIn("_A__mangled", A.f.__code__.co_varnames)
+        self.assertIn("__not_mangled__", A.f.__code__.co_varnames)
+        self.assertIn("_A__mangled_mod", A.f.__code__.co_varnames)
+        self.assertIn("__package__", A.f.__code__.co_varnames)
 
     def test_compile_ast(self):
         fname = __file__