set python_requires in setup.py

[Carreau]

See https://python3statement.org/practicalities/

python_requires will tell to pip ahead of time not to even try to install on incompatible versions. That will avoid error message on triggered by the test in here https://github.com/python-adaptive/adaptive/blob/master/setup.py#L8-L10.

it's too late for already released versions, but at least for the newest ones you could set that up.

[basnijholt]

Thanks a lot! I've addressed this in #180.