update shell.nix and use poetry2nix in default.nix

shell.nix

@@ -3,20 +3,26 @@ in pkgs.mkShell {
   buildInputs = with pkgs; [
     git
     nixfmt
-    pkgs.python37
-    pkgs.python38
-    pkgs.python3
-    pkgs.python3Packages.coveralls
-    pkgs.python3Packages.black
-    pkgs.python3Packages.poetry
-    pkgs.python3Packages.pytest
-    pkgs.python3Packages.pytest-black
-    pkgs.python3Packages.pytestcov
-    pkgs.python3Packages.pytest-flake8
-    pkgs.python3Packages.pytest-isort
-    pkgs.python3Packages.pytest-mypy
-    pkgs.python3Packages.pytest-pylint
-    pkgs.python3Packages.tox
+    gnumake
+    python3
+    python37
+    python37Packages.virtualenv
+    python38
+    python38Packages.virtualenv
+    python39
+    python39Packages.virtualenv
+    python3Packages.black
+    python3Packages.coveralls
+    python3Packages.poetry
+    python3Packages.pytest
+    python3Packages.pytest-black
+    python3Packages.pytestcov
+    python3Packages.pytest-flake8
+    python3Packages.pytest-isort
+    python3Packages.pytest-mypy
+    python3Packages.pytest-pylint
+    python3Packages.sphinx
+    python3Packages.tox
   ];
 
   shellHook = ''