manifest: type and fix unhexlify

Some part of that function seems to date back from Python 2. It raise question
about whether this function is useful or not, but let us just fix it for now.
This was caught by pytype.

mercurial/manifest.py

@@ -143,10 +143,10 b' class lazymanifestiterentries:'
     __next__ = next
 
 
-def unhexlify(data, extra, pos, length):
+def unhexlify(data: bytes, extra: int, pos, length: int):
     s = bin(data[pos : pos + length])
     if extra:
-        s += chr(extra & 0xFF)
+        s += bytes([extra & 0xFF])
     return s