Motivation for this change

Add opentimestamps-client with all Python dependencies

Additional info to help reviewing

pysha3

Dependency of opentimestamps
https://github.com/tiran/pysha3
https://pypi.python.org/pypi/pysha3

opentimestamps

Main library, used by the client
https://github.com/opentimestamps/python-opentimestamps
https://pypi.python.org/pypi/opentimestamps/0.2.1

opentimestamps-client

https://pypi.python.org/pypi/opentimestamps-client/0.5.1
https://github.com/opentimestamps/opentimestamps-client

We can't use the pypi source because it doesn't include README.md which is
needed in setup.py. Fetch the pypi source:

nix-build --no-out-link  -E '(import <nixpkgs> {}).pythonPackages.fetchPypi {
   pname = "opentimestamps-client";
   version = "0.5.1";
   sha256 = "1pcch5fnazaw5dxbh4r0nx5f16xrbky81hgq10gs4hgip9zf9z2a";
}'

Further additions

If you'd welcome it, I could also add opentimestamps-server

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.