default.nix: add python3 to the shell

There are many systems that chose to not have a global python install,
namely my own systems :p. I end up having to nix-shell -p python3 anytime
I want to run the build.sh as the last step uf2conv depends on python

default.nix

@@ -4,6 +4,7 @@ in
   pkgs.mkShell {
     buildInputs = [
       pkgs.platformio
+      pkgs.python3
       # optional: needed as a programmer i.e. for esp32
       pkgs.avrdude
     ];