Updating pytest

[Julian Gilbey <julian@d-and-j.net>]

Hi all,

When I updated pytest-mock, I noticed that pytest is somewhat out of
date and it would be good to upgrade it.  But it's quite a major
package, and I don't really want to do it without a go-ahead from
others.

Perhaps we could upload a newer version to experimental first to see
what breaks?

Best wishes,

   Julian