Cannot use `withPackages`

[asymmetric]

I have the following in my flake.nix:

devShells.default = pkgs.mkShell {

  packages = [
    (inputs.nixpkgs-python.packages.${system}."3.9.2".withPackages (ps: with ps; [ requests ]))
  ]

...
}

which fails to build:

error checking the existence of https://tarballs.nixos.org//:
curl: (6) Could not resolve host: tarballs.nixos.org

trying https://www.python.org/ftp/python/3.9.2/Python-3.9.2.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
^M  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: www.python.org
Warning: Problem : timeout. Will retry in 1 seconds. 3 retries left.
^M  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: www.python.org
Warning: Problem : timeout. Will retry in 2 seconds. 2 retries left.
^M  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: www.python.org
Warning: Problem : timeout. Will retry in 4 seconds. 1 retries left.
^M  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0curl: (6) Could not resolve host: www.python.org
error: cannot download Python-3.9.2.tar.xz from any mirror

If I try adding matplotlib instead, the error is different:

error:
       … while calling the 'derivationStrict' builtin

         at //builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'market-maker-stats'
         whose name attribute is located at /nix/store/46i6y04hr6zq7054mg2svxgk9zzpsiqy-source/pkgs/stdenv/generic/make-derivation.nix:303:7

       … while evaluating attribute 'nativeBuildInputs' of derivation 'market-maker-stats'

         at /nix/store/46i6y04hr6zq7054mg2svxgk9zzpsiqy-source/pkgs/stdenv/generic/make-derivation.nix:347:7:

          346|       depsBuildBuild              = lib.elemAt (lib.elemAt dependencies 0) 0;
          347|       nativeBuildInputs           = lib.elemAt (lib.elemAt dependencies 0) 1;
             |       ^
          348|       depsBuildTarget             = lib.elemAt (lib.elemAt dependencies 0) 2;

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: function 'anonymous lambda' called with unexpected argument 'x11Support'

       at /nix/store/ysisn7pamb4plgr95w5szis24pfwihfd-source/self.nix:1:1:

            1| { nixpkgs-python, version }:
             | ^
            2|