Fix compatibility with Jython
Fix compatibility with Jython

This patch was taken from
https://github.com/yaml/pyyaml/issues/369#issuecomment-571596545,
authored by Pekka Klärck <peke@iki.fi>.

In short, Jython doesn't support lone surrogates, so importing yaml (and
in particular, loading `reader.py`) caused a UnicodeDecodeError. This
patch works around this through a clever use of `eval` to defer
evaluation of the string containing the lone surrogates, only doing it
on non-Jython platforms.

This is only done in `lib/yaml/reader.py` and not `lib3/yaml/reader.py`
because Jython does not support Python 3.

With this patch, Jython's behavior with respect to Unicode code points
over 0xFFFF becomes as it was before
0716ae21a1e7ab6b4ef73428c0c8fff49685d057. It still does not pass all the
unit tests on Jython (passes 1275, fails 3, errors on 1); all the
failing tests are related to unicode. Still, this is better than simply
crashing upon `import yaml`.

With this patch, all tests continue to pass on Python 2 / Python 3.

yaml/pyyaml,anishathalye/pyyaml,anishathalye/pyyaml,yaml/pyyaml,yaml/pyyaml