Handle L1's None nodata values

Bump flake8 to 3.8.2

Restore support for attrs 19.1.0

The 'eq' field needs 19.3.0 or newer.

Clean-up some minor library warnings

Warnings produced while running tests, mostly
deprecations.

Fix the two tests that fail on gdal 3

Which I'm guessing cause the failures in opendatacube#65

The duplication between these two files is a bit messy.

Only include s2 subfolder when s2 naming is chosen

And try to avoid too much product-specific logic in the public api. Other products will also want subfolders.

fmask:cloud should always be included

Even when used by eo:cloud_cover.

Requested in change feedback.

Add link to readthedocs from README.

Clean-up readme to focus on what most people care about

Most people want to use the public APIs, and will want
to install from pip.

Show a document failure only for errors

Informational messages, or warnings when not in
strict mode, should still show a green tick.