mgr/dashboard: fix config in tox/flake8
As a result of the refactoring introduced in https://github.com/ceph/ceph/pull/28696/files now flake8 is used instead of pycodestyle (technically flake8 uses pycodestyle for PEP8 checking). Some misconfigurations should be fixed:
- Reinstate pytest-faulthandler
- Remove ".*" from flake8 excludes (which basically makes flake8 not to do anything).
- Lock requirements-lint versions