gh-94526: getpath_dirname() no longer encodes the path (GH-97645)

Fix the Python path configuration used to initialized sys.path at
Python startup. Paths are no longer encoded to UTF-8/strict to avoid
encoding errors if it contains surrogate characters (bytes paths are
decoded with the surrogateescape error handler).

getpath_basename() and getpath_dirname() functions no longer encode
the path to UTF-8/strict, but work directly on Unicode strings. These
functions now use PyUnicode_FindChar() and PyUnicode_Substring() on
the Unicode path, rather than strrchr() on the encoded bytes string.
(cherry picked from commit 9f2f1dd131b912e224cd0269adde8879799686c4)

Co-authored-by: Victor Stinner <vstinner@python.org>