Setup flake8 CI

[github-actions[bot]]

Description

Setup flake8 CI.

To do

• [x] Add flake8 to the static analysis workflow
• [x] Cache pip installed requirements

References

• Scilpy coding standards
• linum / linumpy / .gitlab-ci.yml
• flake8

Dependencies

• Depends on #19
• Depends on #29 closes #32

[github-actions[bot]]

This PR/issue depends on:

• benoit-dubreuil/inm5803-ete2022#19
• benoit-dubreuil/inm5803-ete2022#29 By Dependent Issues (🤖). Happy coding!