CI: Run pytest, Flake8 and mypy on GitHub Actions on Ubuntu 20.04, 22.04, Debian and macOS

[hoh]

Problem

We would like to support many user systems with aleph-client, including:

• Ubuntu 20.04
• Ubuntu 22.04
• Debian Bullseye
• macOS 12 Monterey
• Windows 10

While most of the code is portable, some dependencies are compiled differently for different systems.

Tests currently run only on Ubuntu 20.04 via a Docker image, as defined in https://github.com/aleph-im/aleph-client/blob/master/.github/workflows/test-pytest.yml

This gives us low confidence on whether aleph-client works on these platforms, and we have no idea if a commit breaks it.

Proposal

Add GitHub Actions workflows that run pytest with flake8 and check for coverage. Also check types using mypy.

https://pypi.org/project/flake8/
https://pypi.org/project/mypy/
https://pypi.org/project/pytest-cov/