[3.10] bpo-45461: Fix IncrementalDecoder and StreamReader in the "unicode-escape" codec (GH-28939) (GH-28943)

They support now splitting escape sequences between input chunks.

Add the third parameter "final" in codecs.unicode_escape_decode().
It is True by default to match the former behavior.
(cherry picked from commit c96d1546b11b4c282a7e21737cb1f5d16349656d)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Lib/encodings/unicode_escape.py b/Lib/encodings/unicode_escape.py
index 817f932..9b1ce99 100644
--- a/Lib/encodings/unicode_escape.py
+++ b/Lib/encodings/unicode_escape.py
@@ -21,15 +21,16 @@ class IncrementalEncoder(codecs.IncrementalEncoder):
     def encode(self, input, final=False):
         return codecs.unicode_escape_encode(input, self.errors)[0]
 
-class IncrementalDecoder(codecs.IncrementalDecoder):
-    def decode(self, input, final=False):
-        return codecs.unicode_escape_decode(input, self.errors)[0]
+class IncrementalDecoder(codecs.BufferedIncrementalDecoder):
+    def _buffer_decode(self, input, errors, final):
+        return codecs.unicode_escape_decode(input, errors, final)
 
 class StreamWriter(Codec,codecs.StreamWriter):
     pass
 
 class StreamReader(Codec,codecs.StreamReader):
-    pass
+    def decode(self, input, errors='strict'):
+        return codecs.unicode_escape_decode(input, errors, False)
 
 ### encodings module API