Use pyflakes for static code checks in CI

[maxnoe]

Is your feature request related to a problem? Please describe.

We made great experience in ctapipe and lstchain runnig pyflakes to catch simple static errors before running the unit test suite.

Describe the solution you'd like

Run pyflakes on gammapy as part of the CI

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context

Currently, pyflakes reports a large number of issues in gammapy:

❯ pyflakes gammapy | wc -l
294

However, most of them are related to star imports which make static checking per file for undefined objects impossible.

We should fix these issues to be able to use pyflakes as part of the CI.