* [nix] Update nixpkgs to avoid issues with Big Sur
The older nixpkgs snapshot did not contain nix changes to the
compiler/linker hooks that are necessary for compatibility with MacOS
Big Sur. The fix is simply to update to a newer snapshot.
* [nix] Add a poetry manifest and use poetry to build the Python env
* [nix] Use niv to manage upstream sources like nixpkgs
* [nix] Update to newer nixpkgs snapshot
* [nix] Bump python package versions
