Include third-party code licenses #2131

Some code in the repo comes from third parties whose licenses require the license text to be included when we redistribute it. For example:

• BouncyCastle hashlib digests (MIT)
• Schubfach float repr (MIT)
• zlib.net (BSD-3-Clause)
• DotNetZip BZip2 (MS-PL; partly derived from Apache Commons, Apache-2.0)
• Unicode Character Database (Unicode License, see also Update unicodedata module #691)
• Other stuff?

We should gather these notices into a file (maybe THIRD-PARTY-NOTICES) and ship it with the release packages.