shell.nix: make more similar to the code in Nixpkgs

Remove the dependency on installShellFiles as it is not needed for building kitty itself and is only used in Nixpkgs for conveniently installing man pages and shell completions.
pkgconfig has been renamed to pkg-config.
Use `with python3Packages;` to reduce code duplication. This is how it's done in Nixpkgs as well.
Also sort the packages in nativeBuildInputs.

shell.nix

@@ -7,6 +7,7 @@ let
   inherit (darwin.apple_sdk.frameworks) Cocoa CoreGraphics Foundation IOKit Kernel OpenGL;
   harfbuzzWithCoreText = harfbuzz.override { withCoreText = stdenv.isDarwin; };
 in
+with python3Packages;
 mkShell rec {
   buildInputs = [
     harfbuzzWithCoreText
@@ -29,17 +30,18 @@ mkShell rec {
   ] ++ checkInputs;
 
   nativeBuildInputs = [
-    pkgconfig python3Packages.sphinx ncurses
+    ncurses
+    pkg-config
+    sphinx
   ] ++ optionals stdenv.isDarwin [
     imagemagick
     libicns  # For the png2icns tool.
-    installShellFiles
   ];
 
   propagatedBuildInputs = optional stdenv.isLinux libGL;
 
   checkInputs = [
-    python3Packages.pillow
+    pillow
   ];
 
   # Causes build failure due to warning when using Clang