Replace pyasn1 with upstream version 0.1.7
pyasn1: use relative imports
Run 2to3
Fix base64 encoding/decoding
xmlsec/__init__.py: use hmac.compare_digest if available

Fall back to the previous behavior of directly checking equality.
Handle str/bytes distinction
xmlsec/__init__.py: don't use DictMixin

It no longer exists.
x509_pem.py: update key parsing

... to match changes in the pyasn1 module
setup.py: require 'six' library
pyasn1: try to return text instead of bytes when pretty-printing
Fix more str/bytes issues
xmlsec/utils.py: use six.moves.html_entities
Fix rsa_x509_pem tests in Python 2
x509_pem.py: fix usage of long integers in Python 2
Fix a few more oddities introduced in 2to3 conversion