Remove pointless print
Modernisation fixes

We're going to support Python 3, so remove things which don't.
Match the signature change in PendingSend

These should ideally have been included when I removed the automated
DB connection in 875c15bfb58a1eff.
Extract the name of the mailer's config root to a constant
Also get the template path from config.
Convert the mailer utils to being a class

Inject the config object and sql connector into this class at
construction, rather than directly accessing an import.

This makes it clearer where it gets its config from, as well as
being much easier to test.
Remove local handling of config

This is a library, and shouldn't need configuring locally like this.
Handle the 'config' as a dictionary for now, though it may make more
sense to have it just as arguments eventually.
Add docstrings to Mailer and send_email