encoding: make wide character class list a sysstr

That's what east_asian_width returns, so just match it.

mercurial/encoding.py

@@ -194,7 +194,7 b' if not _nativeenviron:'
                    for k, v in os.environ.items())  # re-exports
 
 # How to treat ambiguous-width characters. Set to 'wide' to treat as wide.
-wide = (environ.get("HGENCODINGAMBIGUOUS", "narrow") == "wide"
+wide = _sysstr(environ.get("HGENCODINGAMBIGUOUS", "narrow") == "wide"
         and "WFA" or "WF")
 
 def colwidth(s):