style: apply flake8-copyright rules

[DimitriPapadopoulos]

CPY001 Missing copyright notice at top of file

Do not apply this rule to empty files:

• tests/rules/__init__.py

and to files generated by tools:

• docs/conf.py

[coveralls]

coverage: 99.822%. remained the same when pulling 7b6a01726124dfb5036fd8bf462a03c1edbb2dbb on DimitriPapadopoulos:flake8-copyright into 0d7ae7bf6b852fd739ba385b686ea83ed3164a79 on adrienverge:master.

[adrienverge]

OK, thank you.