Ensure that flake8 and flake8-ets are used on all ETS projects

[rahulporuri]

At the moment, not all ETS projects are flake8-clean and not all of them ensure that the copyright headers are up-to-date (using flake8-ets). It would be nice to slowly cleanup the ETS projects and setup CI to ensure that the codebases stay clean.

• [ ] envisage
• [ ] traitsui (doesnt run flake8-ets)
• [ ] pyface
• [ ] enable
• [ ] chaco
• [ ] scimath

[rahulporuri]

Note also that we need to decide if we want all ETS projects to use isort. At the moment, only Traits Futures use isort.