Really only use PyUnicode_FromUnicode() when needed (GH-3697)

* Really only use PyUnicode_FromUnicode() for strings that contain lone surrogate, not for normal non-BMP strings and not for surrogate pairs on 16bit Unicode platforms.

See https://github.com/cython/cython/issues/3678

* Extend buildenv test to debug a MacOS problem.
* Add a test for surrogate pairs in Unicode strings.
* Limit PyUnicode_FromUnicode() usage to strings containing lone surrogates.
* Accept ambiguity of surrogate pairs in Unicode string literals when generated on 16bit Py2 systems.