GitLab

- manual and on tags trigger

- #245: Use own built images with Poetry
- Checks: build, format
- Use Black installed from Docker image v3.8
- tests: Pytest for Python v3.5, 3.6, 3.7 (+ coverage), and 3.8
- Coverage published to GitLab Pages once tests-3.7 job is done

- https://github.com/petercollingridge/SVG-Optimiser

- Converted from the matrix image (PNG) with Adobe Illustrator

- Fix Transaction document generation

- generic f() to retrieve identities from a list of pubkeys

- Implements equality and add time attribute to Transaction class