py3: cast character set to bytes

The returned value from this function is eventually fed into
email.charset.Charset.__init__. However, the standard library
appears to handle both string types in Python 2 and 3. So there
shouldn't be any problem returning a bytes here.

Differential Revision: https://phab.mercurial-scm.org/D2167

mercurial/mail.py

@@ -187,7 +187,7 b' def validateconfig(ui):'
 
 def codec2iana(cs):
     ''''''
-    cs = email.charset.Charset(cs).input_charset.lower()
+    cs = pycompat.sysbytes(email.charset.Charset(cs).input_charset.lower())
 
     # "latin1" normalizes to "iso8859-1", standard calls for "iso-8859-1"
     if cs.startswith("iso") and not cs.startswith("iso-"):