pytrainer: unpin python 3.10

The `websockets` library, which is a transitive dependency does not
build with Python 3.10 any more. Switching to the default Python
version fixes the build.

pkgs/applications/misc/pytrainer/default.nix

@@ -1,5 +1,5 @@
 { lib
-, python310
+, python3
 , fetchFromGitHub
 , gdk-pixbuf
 , adwaita-icon-theme
@@ -17,7 +17,7 @@
 }:
 
 let
-  python = python310.override {
+  python = python3.override {
     self = python;
     packageOverrides = (self: super: {
       matplotlib = super.matplotlib.override {