This was fixed in December in the current tip: rev 6295:bc2b00afa980
and will be in the next release of roundup.

You can edit roundup-server and replace base64.decodestring with 
base64.b64decode which works on both 2.7 and 3.x.

Diff of the commit:

diff -r 48a0f724ffbc -r bc2b00afa980 roundup/scripts/roundup_server.py
--- a/roundup/scripts/roundup_server.py Sun Nov 29 16:11:22 2020 -0500
+++ b/roundup/scripts/roundup_server.py Mon Dec 14 09:52:58 2020 -0500
@@ -74,7 +74,7 @@
 # "default" favicon.ico
 # generate by using "icotool" and tools/base64
 import zlib, base64
-favico = zlib.decompress(base64.decodestring(b'''
+favico = zlib.decompress(base64.b64decode(b'''
 
eJztjr1PmlEUh59XgVoshdYPWorFIhaRFq0t9pNq37b60lYSTRzcTFw6GAfj5gDYaF0dTB0M
xMSE
 
gQQd3FzKJiEC0UCIUUN1M41pV2JCXySg/0ITn5tfzvmdc+85FwT56HSc81UJjXJsk1UsNcsS
qCk1
 
BS64lK+vr7OyssLJyQl2ux2j0cjU1BQajYZIJEIwGMRms+H3+zEYDExOTjI2Nsbm5iZWqxWv
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diff -r 48a0f724ffbc -r bc2b00afa980 
roundup/scripts/roundup_xmlrpc_server.py
--- a/roundup/scripts/roundup_xmlrpc_server.py  Sun Nov 29 16:11:22 2020 
-0500
+++ b/roundup/scripts/roundup_xmlrpc_server.py  Mon Dec 14 09:52:58 2020 
-0500
@@ -64,7 +64,7 @@
         scheme, challenge = authorization.split(' ', 1)

         if scheme.lower() == 'basic':
-            decoded = base64.decodestring(challenge)
+            decoded = base64.b64decode(challenge)
             if ':' in decoded:
                 username, password = decoded.split(':')
             else:

Have a great week.

-- rouilj