NamedTemporaryFile supports the "delete" argument only since 2.6.

Defer is_raw_json test

Instances of RawJSON are unusual; by moving the test for them to lower in
encoder_listencode_obj, we get a 12% performance improvement in my tests by not
doing the instanceof test.

Avoid escaping U+2028 and U+2029 without ensure_ascii

* `\b\f\n\r\t` are all in the range `\x00-\x1f` so can be omitted from ESCAPE
* `ESCAPE_DCT[0x2028:0x2029]` is no longer used in `encode_basestring`, and
  `py_encode_basestring_ascii` handles them via the generic `\uNNNN` code, so
  these entries can be removed from `ESCAPE_DCT`.

Document the U+2028 and U+2029 behavior.

Now that we do not include \uNNNN escapes in the regex, we can use a raw string
rather than having to double-escape everything.