Introduce Nox for Test Automation

This MR introduces the test automation tool nox to pystencils-sfg. At this point, the nox configuration offers these sessions:

• lint, for running flake8 on the codebase
• typecheck, for running mypy
• testsuite, to run the testsuite and collect coverage data
• docs to build the documentation pages
• dev_env, to quickly set up a virtual environment for development

Through Nox, the appropriate pre-release version of pystencils 2.0 will be dynamically loaded for testing.

The GitLab CI is adapted to also use Nox. For the time being, it runs in a minimal Alpine Linux-based container featuring Python 3.10, nox, and the necessary C++ build tools. This will have to be extended in the near future, e.g. with CUDA/HIP support.

Depends on !11 (merged)