Python-darc

* bugfix for docker
* revised extraction of links from text
* added support for ETH
* revised data file output (as JSON lines)
* added backref for link extracted
* minor bugfix in docs

* revised typing annotations (improved its usage & accelerate the loading process)
* updated demo/sample/extra scripts
* updated dependencies (Python & binary)
* minor bugfix around the main project
* bumped Docker dependency from bionic to focal & Python 3.8 to Python 3.9
* introducted Redlock from Pottery for real ditributed Redis locks

* LinkNoReturns now accepts link and drop parameters
* minor type checks on source code
* added some samples
* updated docs
* added GitLab CI (for mirrored repo)