From now on git2graph only exports edges and allow to filter on them. Nodes
should be generated with the usual "| tr | sort -u" trick, which is now
documented in README. Filering on nodes is only induced by edge filtering.

using bats as unit test framework

This allows to select desired nodes and edges based on their types, similarly
to what the edges restriction field in the swh-graph REST API does. (It is in
fact strictly more expressive, as one can also select nodes without the
corresponding edges.)

The support is only static for now: boolean arrays (or matrices) have to be
changed in the source code to alter the selection, which by default is "all".
Support for making this user-selectable on the CLI is upcoming.

no other content changes

hat tip to @olasd for the tips

- drop the hardcoded (and duplicated) version number from pom.xml, using a
  git-describe based plugin
- drop the version number from the *.jar file, which is now always called
  "swh-graph-jar-with-dependencies.jar"
- switch Dockerfile from oracle linux to Debian buster

this makes mypy pass again on swh-graph

this considerably speed up this (silly) job

make mypy 0.730 pass cleanly again

this avoid duplication in the Dockerfile and also allows override from the
command-line

This was probably broken around a428cd8c and
caused SLF4J logging configuration to be ignored, falling back to crazye DEBUG
logging in the MPH step.