Fix #599 by allowing subclasses of string and unicode in sys.path

[KoykL]

I use isSubclass check if a given entry in sys.path is subclass of str or unicode and tried to allow both situations. If it is unicode, I also convert it to str with PyUnicode_AsEncodedString() with Py_FileSystemDefaultEncoding.

I also modified nullImporterInit to make it accept subclasses of str.

[kmod]

Looks great! Can we get a test though? If you drop a file in test/tests/, it will get run and have its output compared against CPython's. Since we have a file called test/tests/test_package/import_target.py, we could test this with something like

import sys
sys.path.append(u"test_package")
import import_target

[KoykL]

Hi! I add four individual tests with prefix of sys_path_ for four possible types of entries in sys.path, namely str, unicode, subclass of str, and subclass of unicode.

[undingen]

LGTM, thanks for the patch!
Sorry for the late response but kmod and me where both on holiday.