Remove references to python 3.6 and earlier.

This also updates dependencies and ensures the build remains functional.
Some minor reordering occurred when updating lists of dependencies as
well.

nix/default.nix

@@ -3,13 +3,4 @@ let
 in
   pkgs.poetry2nix.mkPoetryApplication {
     projectDir = ../.;
-    overrides = pkgs.poetry2nix.overrides.withDefaults (self: super: {
-      pytest = super.pytest.overrideAttrs (old: {
-        postPatch = old.postPatch or "" + ''
-          sed -i '/\[metadata\]/aversion = ${old.version}' setup.cfg
-          echo "VERSION"
-          grep version setup.cfg
-        '';
-      });
-    });
   }

nix/shell.nix

@@ -5,10 +5,10 @@ in
   pkgs.mkShell {
     inputsFrom = [zfs-replicate];
     buildInputs = [
-      zfs-replicate
       pkgs.poetry
-      pkgs.pre-commit
+      pkgs.python38Packages.pre-commit
       pkgs.python38Packages.virtualenv
+      zfs-replicate
     ];
     shellHook = ''
       unset SOURCE_DATE_EPOCH