py/objstr: startswith, endswith: Check arg to be a string.

Otherwise, it will silently get incorrect result on other values types,
including CPython tuple form like "foo.png".endswith(("png", "jpg"))
(which MicroPython doesn't support for unbloatedness).

tests/basics/string_startswith.py

@@ -9,3 +9,8 @@ print("1foo".startswith("foo", 1))
 print("1foo".startswith("1foo", 1))
 print("1fo".startswith("foo", 1))
 print("1fo".startswith("foo", 10))
+
+try:
+    "foobar".startswith(1)
+except TypeError:
+    print("TypeError")