Update urllib3 dependency version

[alcaeus]

Installing requirements as suggested in the docs leads to the following error:

requests 2.22.0 requires urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1, but you'll have urllib3 1.26.5 which is incompatible.

Downgrading the urllib3 dependency in this file fixes the error and allows docs builds again.