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_endswith.py

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