Upgrade the encryption algorithm on another thing. This uses the same PBES2 parameters as the previous one.

If someone has a choice for a 2nd algorithm, I'm happy to do that. See previous PR for ideas about how to review
Update docs
Merge pull request #1484 from alex/re-encode-thing

Upgrade the encryption algorithm on another thing. This uses the same PBES2 parameters as the previous one.
Merge branch 'master' into pure-python-pem
Initial work on PBES2