This commit introduces REUSE compliance by annotating all files
with SPDX information and placing the reused licences in the
LICENSES folder.
We additionally removed the docheader tool which is made obsolete
by this change.
The main LICENSE and copyright text of the project is now not under
my personal name anymore, and it belongs to "The Respect Project
Contributors" instead.
This change restores author names to several files, giving the
appropriate attribution for contributions.
The `Validator` class implements the Builder patterns, because it builds
a complex validator within a chain. This is a major breaking change, as
the `Validator` class is the foundation of the library. However, that’s
something relatively easy to replace everywhere.
This is a pattern that we already use everywhere, so it makes sense to
simply turn `v` into a class alias to `Validator`, which will allow
people to use all the examples in the documentation without needing to
import the namespace.
We already do that when we run tests, so I just changed it to be a
global setting.
