bin/sync_members throws UnicodeDecodeError with non-ascii names.

If bin/sync_members is adding a member whose display name includes non-ascii, it can throw a UnicodeDecodeError as in

Traceback (most recent call last):
  File "/usr/local/mailman/bin/sync_members", line 288, in <module>
    main()
  File "/usr/local/mailman/bin/sync_members", line 259, in main
    s = email.Utils.formataddr((name, addr)).encode(enc, 'replace')
UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 3: ordinal not in range(128)

A similar issue may occur with removal.