Update pyshell template

Use derivation dependencies instead of derivation itself by default.

Set matplotlib backend via MPLBACKEND environment variable.

templates

@@ -199,12 +199,13 @@ nix-mode
          > "(python3.withPackages(" n
          > "ps: with ps; ["n
          > (p "tkinter mypy") n
-         > (p "(callPackage ./derivation.nix { })") n
          > "]" n
+         > (p "++ (callPackage ./derivation.nix { }).dependencies") n
          > "))" n
          > "];" n
          > "shellHook = ''" n
          > "export PYTHONPATH=$(readlink -f " (p "./") "):$PYTHONPATH" n
+         > "export MPLBACKEND=" (p "TkAgg") n
          > "'';"n
          > "}")