Install pre-commit and fix configuration and builds

[ziotom78]

This small PR installs pre-commit and documents its usage. This module is optional for developers, but it can save a lot of time when doing modifications to the repository. Now that issue #126 has been fixed, all the tests in bin/run_tests.sh are ran and checked properly by GitHub, and things are more likely to go bad! The script checks for proper code formatting and a number of stylistic errors (e.g., too long lines, unused variables).

The usage of pre-commit enables developers to discover these problems before running git push, as black and flake8 are ran automatically whenever the user calls git commit.

There is no need for developers to use this facility, so if you're happy with your workflow, this PR does not affect it.

[ziotom78]

I have removed the AppVeyor config file, which was used only to check that the code was runnable under WSL but it has not been maintained since ages. We'll decide if we want to add tests for Windows using GitHub Actions, which would be far easier to handle.