Jsonrpcserver

- Fix file permission on all files.

- Add InvalidParamsError exception, for input validation. Previously the
advice was to `assert` on input values. However, AssertionError was too
generic an exception. Instead, raise InvalidParamsError. Note `assert` will
still work but will be removed in the next major release (5.0).
- Add an ApiError exception; raise it to send an application defined error
response. This covers the line in the JSON-RPC spec, "The remainder of the
space is available for application defined errors."
- A KeyError raised inside methods will no longer send a "method not found"
response.
- Uncaught exceptions raised inside methods will now be logged. We've been
simply responding to the client with a Server Error. Now the traceback will
also be logged server-side.
- Fix a deprecation warning related to collections.abc.
- Add py.typed to indicate this package supports typing. (PEP 561)

Thanks to steinymity for his work on this release.

- Include license in package.

- Use faster method of jsonschema validation
- Use inspect from stdlib, removing the need for funcsigs

- Update dependencies to allow jsonschema version 3.x
- Support Python 3.8

- Fix to allow passing context when no parameters are passed.