BUG: File I/O needs bytes in Python 3

The wrapper that allows us to use Python file-like objects as C++
streams was specific to Python 2 inasmuch as it wrapped data in a
str() before sending it to the write() method.

As of Boost 1.67, Boost.Python doesn't have a convenient wrapper for
bytes the way it does str.  We instantiate a bytes() object around the
raw data ourselves.

Some of the issues involved are discussed in this pull request:

boostorg/python#54

tracktable/PythonWrapping/PythonFileLikeObjectStreams.h

@@ -137,11 +137,17 @@ class PythonWriteSink
     // can do is dump the data out, claim it was all written and
     // hope for the best.
 
+#if PY_MAJOR_VERSION == 2
     boost::python::str data(buffer, n);
 
     boost::python::object write_result(this->Writer(data));
     boost::python::extract<std::streamsize> bytes_written(write_result);
-
+#else
+    boost::python::object data(boost::python::handle<>(PyBytes_FromStringAndSize(buffer, n)));
+    boost::python::object write_result(this->Writer(data));
+    boost::python::extract<std::streamsize> bytes_written(write_result);
+#endif
+
     return (bytes_written.check() ? bytes_written() : n);
   }