python311.pkgs.pip: fix eval

Sphinx bumped their minimum supported python from 3.11 to 3.12, but this
condition wasn't updated in the pip package accordingly.

Instead of just bumping here, let's reuse the sphinx.disabled condition
so that we don't need to worry about it in the future.

(cherry picked from commit f961efa543)

pkgs/development/python-modules/pip/default.nix

@@ -29,6 +29,9 @@
 }:
 
 let
+
+  sphinxSupported = !sphinx.disabled;
+
   self = buildPythonPackage rec {
     pname = "pip";
     version = "25.3";
@@ -51,9 +54,8 @@ let
       flit-core
       installShellFiles
     ]
-    ++ lib.optionals (pythonAtLeast "3.11") [
+    ++ lib.optionals sphinxSupported [
       # docs
-      # (sphinx requires Python 3.11)
       sphinx
       sphinx-issues
     ];
@@ -61,13 +63,13 @@ let
     outputs = [
       "out"
     ]
-    ++ lib.optionals (pythonAtLeast "3.11") [
+    ++ lib.optionals sphinxSupported [
       "man"
     ];
 
     # pip uses a custom sphinx extension and unusual conf.py location, mimic the internal build rather than attempting
     # to fit sphinxHook see https://github.com/pypa/pip/blob/0778c1c153da7da457b56df55fb77cbba08dfb0c/noxfile.py#L129-L148
-    postBuild = lib.optionalString (pythonAtLeast "3.11") ''
+    postBuild = lib.optionalString sphinxSupported ''
       cd docs
 
       # remove references to sphinx extentions only required for html doc generation