python38Packages: build packages on Hydra

Now that Python 3.8 is released we will also build packages on Hydra.
As soon as the far majority of packages build, we can make Python 3.8
the default `python3`.

https://github.com/NixOS/nixpkgs/issues/71220

pkgs/top-level/all-packages.nix

@@ -7294,7 +7294,7 @@ in
   zsh-git-prompt = callPackage ../shells/zsh/zsh-git-prompt { };
 
   zsh-history = callPackage ../shells/zsh/zsh-history { };
-  
+
   zsh-history-substring-search = callPackage ../shells/zsh/zsh-history-substring-search { };
 
   zsh-navigation-tools = callPackage ../tools/misc/zsh-navigation-tools { };
@@ -8960,7 +8960,7 @@ in
   python35Packages = python35.pkgs;
   python36Packages = python36.pkgs;
   python37Packages = recurseIntoAttrs python37.pkgs;
-  python38Packages = python38.pkgs;
+  python38Packages = recurseIntoAttrs python38.pkgs;
   pypyPackages = pypy.pkgs;
   pypy2Packages = pypy2.pkgs;
   pypy27Packages = pypy27.pkgs;