Update copyright year in the docs automatically

https://github.com/pytest-dev/pytest/blob/2612d967f8da5265287231f9a10b4d653c78ff13/doc/en/conf.py#L57

We can use the same approach taken in python-trio/trio#465 and use a range:

```python
year = datetime.datetime.utcnow().year
copyright = '2017–{}, Nathaniel J. Smith'.format(year)
```