* [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
* Pin avr-gcc in shell.nix pending release of 8.3.0
There's apparently a critical bug in 8.2.0, which is now the nixpkgs default. This change overrides that default in favor of the known good version. Once 8.3.0 is the default, the override can be dropped.
* Arch/Manjaro fix