gopy: hardcode python2 for now in tests

This CL makes sure we are using python2 for the tests.
Eventually, we'll have to test both.

main_test.go

@@ -162,7 +162,7 @@ func testPkgWithCFFI(t *testing.T, table pkg) {
 	}
 
 	buf := new(bytes.Buffer)
-	cmd = exec.Command("python", "./test.py")
+	cmd = exec.Command("python2", "./test.py")
 	cmd.Dir = workdir
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = buf