Improve licensing and copyright information

[mxmehl]

In #269 I promised that I would make proposals to improve the licensing and copyright information delivered in chardet.
My proposal is based on REUSE, the gold standard for annotating code with this information.

This pull request makes some first steps as I would first like to gather feedback by maintainers and contributors.

• Use valid SPDX identifier for Python package metadata. Currently, chardet's license is shown as "non-standard" e.g. on deps.dev
• Provide license file in REUSE standard location
• Add SPDX information for license, copyright, and contributors for all files that already had some licensing/copyright information
• Add AUTHORS.rst listing known contributors and some background for the original authors and maintainers.

Now, I'd like to discuss with you whether and how to proceed. In 101497d I make a proposal for drastically reducing the license blurb for two files. It depends on your liking how much boilerplate text about the license you would like to keep.

Also, there are a number of other files that are not equipped with licensing and copyright information. Depending on your feedback I can annotate these remaining files with the default license and copyright text, referring to AUTHORS.rst.

Please also let me know if you know that certain files (or parts of files) are from a different copyrightz holder or even under a different license.