Introduce flake8 for linting and code quality checks

[deveshidwivedi]

This PR introduces Flake8 as a linting tool for Software Discovery Tool to help enforce code style guidelines and catch common errors. Fixes #117

[vivkong]

Looks good to me. Thanks.
FYI @pleia2

[rachejazz]

We'd need to add to documentation about installing pre-commit for this to work! In the installation doc, dependency installing section :)

[deveshidwivedi]

Should I update the docs in this PR itself? @rachejazz @pleia2