As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

'strip_node_info' would be useful for multiple plugins. We should
therefore allow users to use it.

This is only used following key invalidation

All of the errors which subclass from BstError have their first
positional argument as message, LoadError should follow this
ordering for consistency.

This makes the 'Node' API public, and available for use directly for
plugins.

Some call places do not need calls to 'node_sanitize' anymore, therefore
removing the call entirely.

Other still use it for convenience, but that doesn't seem the right way
to do it for consistency. Those places have been replaced by calls to
'Node.strip_node_info()'.

Remove completely '_yaml.dump()' and replace all notions and call by
'roundtrip_dump'