py3: convert the mode argument of os.fdopen to unicodes

Couple of these from the earlier series got lost while rebasing. So this patch
converts them again.

mercurial/debugcommands.py

@@ -910,7 +910,7 b' def debuginstall(ui, **opts):'
 
     def writetemp(contents):
         (fd, name) = tempfile.mkstemp(prefix="hg-debuginstall-")
-        f = os.fdopen(fd, "wb")
+        f = os.fdopen(fd, pycompat.sysstr("wb"))
         f.write(contents)
         f.close()
         return name

mercurial/util.py

@@ -73,7 +73,7 b' def isatty(fp):'
 # destined stdout with a pipe destined stdout (e.g. pager), we want line
 # buffering
 if isatty(stdout):
-    stdout = os.fdopen(stdout.fileno(), 'wb', 1)
+    stdout = os.fdopen(stdout.fileno(), pycompat.sysstr('wb'), 1)
 
 if pycompat.osname == 'nt':
     from . import windows as platform