v0.8.1

Bug fixes:

• Fixed a Python2 -> Python3 migration issue when dealing with dictionary values (thanks @sbrass).

Internal changes:

• Replaced legacy scikit-hep>=0.5.0, <0.6.0 requirement (underused), by the vector package.
• Replaced legacy rootproject/root-ubuntu Docker base image by rootproject/root equivalent.

Documentation:

• Fixed readthedocs automatic documentation building (broken since July 1st 2020).
• Define explicit .readthedocs.yml file with the desired configuration.
• Define GitHub docs.yml workflow, to validate documentation building upon master branch pushes / PRs.