Include UTC timezone with returned datetimes

**Why is this feature valuable to you? Does it solve a problem you're having?**

Datetimes returned by the client library do not include a timezone. As per [lost timezone](https://github.com/dropbox/dropbox-sdk-python/issues/64), the timezone is always UTC.

However, I had to search the internet and find this bug to confirm these times were UTC. I'm also going to have to manually set the timezone to UTC to actually use these datetimes in my application - a datetime without a timezone is basically meaningless.

**Describe the solution you'd like**

It should be trivial to include the UTC timezone everywhere that datetimes are deserialised.

**Describe alternatives you've considered**

N/A, there's not really other alternatives.

**Additional context**

N/A