Simplified and correct direnv and nix-shell environment.

This simplifies the configuration by keeping everything in the
pyproject.yaml and just reading that environment into the nix-shell with
poetry2nix.  This should keep everything much more sane for python
projects and I'll verify this is correct with another project shortly.

.envrc

@@ -3,6 +3,3 @@ watch_file nix/shell.nix
 watch_file nix/default.nix
 watch_file pyproject.toml
 watch_file poetry.lock
-
-# Used by pre-commit
-layout python3

nix/shell.nix

@@ -1,12 +1,13 @@
 let
   pkgs = import <nixpkgs> {};
-  cron-install = import ./default.nix;
+  zfs-replicate = import ./default.nix;
 in
   pkgs.mkShell {
     buildInputs = [
-      cron-install
-      pkgs.poetry
-      pkgs.python3
-      pkgs.python37Packages.virtualenv
+      zfs-replicate
+      (pkgs.poetry2nix.mkPoetryEnv { projectDir = ../.; })
     ];
+    shellHook = ''
+      unset SOURCE_DATE_EPOCH
+    '';
   }

pyproject.toml

@@ -46,16 +46,18 @@ packages = [
 
 [tool.poetry.dependencies]
 click = "^7.1.2"
-pre-commit = "^2.7.1"
 python = "^3.6.1"
 stringcase = "^1.2.0"
 
 [tool.poetry.dev-dependencies]
 coveralls = "^2.1.1"
 hypothesis = "^5.37.3"
+pip = "^20.2.4"
+pre-commit = "^2.7.1"
 pytest = "^6.1.1"
 pytest-cov = "^2.10.0"
 vulture = "^2.0"
+virtualenv = "^20.1.0"
 
 [tool.poetry.scripts]
 zfs-replicate = "zfs.replicate.cli.main:main"