Remove use of datetime.utcnow()

[adamchainz]

Fixes #799.

This needs further testing to check whether the issue from #400 is fixed in the core agent or if it needs handling here, by changing the formatting to remove timezone info.

I no longer have a working dev setup for this though, so I will leave that testing in your hands. (I used to be the maintainer of this package.)

[lancetarn]

Thank you for this, Adam. And for all your work here and with Django. We'll run it through the test apps... which you have also contributed. This is one of those changes that my soul knows is good for python but is not exactly going to be painless. We will schedule a look at the core agent to check out options.