[nix] Use niv and poetry2nix to more easily manage dependencies (#11181)

* [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

nix/sources.json

@@ -0,0 +1,26 @@
+{
+    "niv": {
+        "branch": "master",
+        "description": "Easy dependency management for Nix projects",
+        "homepage": "https://github.com/nmattia/niv",
+        "owner": "nmattia",
+        "repo": "niv",
+        "rev": "af958e8057f345ee1aca714c1247ef3ba1c15f5e",
+        "sha256": "1qjavxabbrsh73yck5dcq8jggvh3r2jkbr6b5nlz5d9yrqm9255n",
+        "type": "tarball",
+        "url": "https://github.com/nmattia/niv/archive/af958e8057f345ee1aca714c1247ef3ba1c15f5e.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    },
+    "nixpkgs": {
+        "branch": "nixpkgs-unstable",
+        "description": "Nix Packages collection",
+        "homepage": "",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "c0e881852006b132236cbf0301bd1939bb50867e",
+        "sha256": "0fy7z7yxk5n7yslsvx5cyc6h21qwi4bhxf3awhirniszlbvaazy2",
+        "type": "tarball",
+        "url": "https://github.com/NixOS/nixpkgs/archive/c0e881852006b132236cbf0301bd1939bb50867e.tar.gz",
+        "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
+    }
+}