Upgrade to pywdns revsion 0.8.0.

pywdns (0.8.0)

• Add str_to_rcode and str_to_name_case functions
• Add str_to_name_case function
• Add str_to_rcode function
• Add str_to_rrclass and str_to_rdata functions from wdns
• domain_to_str(): PyString_FromStringAndSize -> PyString_FromString
  
  this fixes a regression introduced in d04d3919. wdns_domain_to_str() does not return the number of bytes written to dst, it returns the number of bytes read from src. dst is guaranteed to be NUL-terminated after a call to wdns_domain_to_str(), so switch back to using PyString_FromString() to convert dst to a python string.
• the comparison against WDNS_MAXLEN_NAME should be strictly greater than, not greater than or equal to.
• setup.py: add back missing 'import os'

Sponsored by: Farsight Security, Inc.