--- a/mercurial/hgweb/request.py	Thu Nov 08 20:04:07 2018 -0500
+++ b/mercurial/hgweb/request.py	Fri Nov 09 23:49:39 2018 +0000
@@ -540,6 +540,12 @@
             yield self._bodybytes
         elif self._bodygen:
             for chunk in self._bodygen:
+                # PEP-3333 says that output must be bytes. And some WSGI
+                # implementations enforce this. We cast bytes-like types here
+                # for convenience.
+                if isinstance(chunk, bytearray):
+                    chunk = bytes(chunk)
+
                 yield chunk
         elif self._bodywillwrite:
             self._bodywritefn = write