MAINT: Update copyright year in `docs/source/conf.py`

https://github.com/QuantEcon/QuantEcon.py/blob/6fd8955b8c2ded3bcc8032bd61e2a13c4afcfa43/docs/source/conf.py#L104 What NumPy does: https://github.com/numpy/numpy/blob/bd5cb2a293c02b8a50864cec1a751afc54f2ff47/doc/source/conf.py#L121-L122 ```python year = datetime.now().year copyright = f'2008-{year}, NumPy Developers' ```