Wayback

This release downgrades the minimum Python version to 3.6! You can now use
Wayback in places like Google Colab.

The `from_date` and `to_date` arguments for `wayback.WaybackClient.search` can now be `datetime.date` instances in addition to `datetime.datetime`.

Huge thanks to edsu for implementing both of these!

When errors were raised or redirects were involved in `WaybackClient.get_memento()`, it was previously possible for connections to be left hanging open. Wayback now works harder to make sure connections aren't left open.

This release also updates the default user agent string to include the repo URL. It now looks like: `wayback/0.2.2 (+https://github.com/edgi-govdata-archiving/wayback)`.