(related to #531)

Two more updates are needed for GRASS GIS 8.4 compatibility: the grass-session pip package is to be removed as it is incompatible and outdated.

On GRASS-side, this PR has implemented it:
OSGeo/grass#3819

In actinia, here changes are needed (maybe more):

• actinia-core/docker/actinia-core-alpine/Dockerfile

  Line 18 in 4c8b9cf

  RUN pip install --upgrade pip six grass-session --ignore-installed six

• https://github.com/actinia-org/actinia-docker/blob/d2e32a5e2cbfdfe9499d75ebae4e2c7631056116/actinia-alpine/Dockerfile#L45