ui: construct _keepalnum list in a python3-friendly way

It'll be more expensive, but it preserves the behavior.

mercurial/ui.py

@@ -39,7 +39,12 b' from . import ('
 urlreq = util.urlreq
 
 # for use with str.translate(None, _keepalnum), to keep just alphanumerics
-_keepalnum = ''.join(c for c in map(chr, range(256)) if not c.isalnum())
+if pycompat.ispy3:
+    _bytes = [bytes([c]) for c in range(256)]
+    _notalnum = [s for s in _bytes if not s.isalnum()]
+else:
+    _notalnum = [c for c in map(chr, range(256)) if not c.isalnum()]
+_keepalnum = ''.join(_notalnum)
 
 samplehgrcs = {
     'user':