plugins/lsp/pylsp: Add python option to generate obtain the pylsp from.

Currently, pkgs.python3Packges is used by default and is unable to be
changed. This stops consumers from using different/newer python
versions.

plugins/lsp/language-servers/pylsp.nix

@@ -10,10 +10,16 @@ let
   cfg = config.plugins.lsp.servers.pylsp;
 in
 {
+  options.plugins.lsp.servers.pylsp = {
+    pythonPackage = mkOption {
+      type = lib.types.package;
+      description = "Python package to use for lsp.";
+      default = pkgs.python3;
+    };
+
     # All settings are documented here:
     # https://github.com/python-lsp/python-lsp-server/blob/develop/CONFIGURATION.md
-
-  options.plugins.lsp.servers.pylsp.settings = {
+    settings = {
       configurationSources = mkOption {
         type = lib.types.nullOr (
           types.enum [
@@ -491,6 +497,7 @@ in
         '';
       };
     };
+  };
 
   config = mkIf cfg.enable {
     # WARNING: tricky stuff below:
@@ -540,7 +547,7 @@ in
               ))
             )
             (
-              with pkgs.python3Packages;
+              with cfg.pythonPackage.pkgs;
               {
                 pylsp_mypy = pylsp-mypy.overridePythonAttrs (old: {
                   postPatch =
@@ -598,7 +605,7 @@ in
       in
       mkDefault (
         # This is the final default package for pylsp
-        pkgs.python3Packages.python-lsp-server.overridePythonAttrs (old: {
+        cfg.pythonPackage.pkgs.python-lsp-server.overridePythonAttrs (old: {
           propagatedBuildInputs = pylspPlugins ++ old.dependencies;
           disabledTests = (old.disabledTests or [ ]) ++ [
             # Those tests fail when third-party plugins are loaded