Fix a buch of shallow test failures.
Note: in test_fileinput.py, two tests are disabled until I figure out how
to replace these.
diff --git a/Modules/_fileio.c b/Modules/_fileio.c
index 660402f..76b47e5 100644
--- a/Modules/_fileio.c
+++ b/Modules/_fileio.c
@@ -705,7 +705,7 @@
 static PyObject *
 get_mode(PyFileIOObject *self, void *closure)
 {
-	return PyString_FromString(mode_string(self));
+	return PyUnicode_FromString(mode_string(self));
 }
 
 static PyGetSetDef fileio_getsetlist[] = {