Add CI check for backend / python code

[jlesquembre]

We should add some basic checks on CI for the python code (code formatting, linting, ....). See https://github.com/tweag/nixpkgs-graph-explorer/pull/61#pullrequestreview-1374734751 and https://github.com/tweag/nixpkgs-graph-explorer/pull/61#issuecomment-1499079108 for ideas.

[GuillaumeDesforges]

• [ ] formatting
• [ ] linting