platformio: 3.6.6 -> 4.0.3

[peterhoeg]

Motivation for this change

Also move to py3. We have provisioning for overriding python packages which was needed for pio3, which is no longer the case as upstream fixed support for click v6, but I am leaving it in, in case we need another override again (likely).

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @mog @makefu

[mmahut]

@GrahamcOfBorg build platformio