Reported by Nat Dunn on docs@

PyUnicode_AsEncodedString() instead of calling the encode() method of the
host, to handle correctly custom string with an encode() method which doesn't
return a byte string. The encoder of the IDNA codec is now called directly
instead of calling the encode() method of the string.

Patch from Shiyao Ma.

Patch by Elvis Pranskevichus.