Add black-compatible presets for flake8

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

.flake8

+# make flake8 work well with black
+# https://github.com/psf/black/blob/a4014668898f6eb80a9197fc185cac9233f0e68a/docs/compatible_configs.md#configuration-1
+[flake8]
+max-line-length = 88
+extend-ignore = E501