os: listdir(), walk(): Handle unicode strings properly.

CPython lib reference specifies that listdir() should accept both bytes and
str argument, and return value type should match the argument. But no such
stipulation is made for walk(), so we just return strings.

os/os/__init__.py

@@ -115,13 +115,13 @@ def ilistdir_ex(path="."):
         yield dirent
 
 def listdir(path="."):
-    is_bytes = type(path) is bytes
+    is_str = type(path) is not bytes
     res = []
     for dirent in ilistdir_ex(path):
-        fname = str(dirent[4].split('\0', 1)[0], "ascii")
-        if fname != "." and fname != "..":
-            if is_bytes:
-                fname = fsencode(fname)
+        fname = dirent[4].split(b'\0', 1)[0]
+        if fname != b"." and fname != b"..":
+            if is_str:
+                fname = fsdecode(fname)
             res.append(fname)
     return res
 
@@ -130,12 +130,12 @@ def walk(top, topdown=True):
     dirs = []
     for dirent in ilistdir_ex(top):
         mode = dirent[3] << 12
-        fname = str(dirent[4].split('\0', 1)[0], "ascii")
+        fname = dirent[4].split(b'\0', 1)[0]
         if stat_.S_ISDIR(mode):
-            if fname != "." and fname != "..":
-                dirs.append(fname)
+            if fname != b"." and fname != b"..":
+                dirs.append(fsdecode(fname))
         else:
-            files.append(fname)
+            files.append(fsdecode(fname))
     if topdown:
         yield top, dirs, files
     for d in dirs: