Serpent

- support for python 3.11 added (`__getstate__` behavior change)
- replaced travis CI by Github actions

Added "bytes_repr" option to serializer to serialize byte arrays into their Python bytes repr escaped form.
(also added BytesNode to the parser where it directly deserializes into a byte array)

This can be more convenient than having to deal with base-64 encoded bytes, but the serialized data is usually about twice as large and it is slower to process.

dependency/version tweaks

new .net release only:

the assembly is now strong-name signed to be able to reference it from other strong-name signed assemblies, most notably, the new Razorvine.Pyrolite version

Just an update to the Python version
- serpent itself didn't really change
- the souce dist archive now contains the unit tests as well
- the setup.py script was improved to run these tests via its test command
- the travis CI config was updated

- fixed serialization of unicode elements in dicts and sets (this was a regression on Python 2.x)