python/hooks: restore catchConflictHook for python<3.10

By restoring and diverting to the old version.

Previously the newer language features and use of more modern stdlib
imports broke the hook on Python<3.10.

pkgs/development/interpreters/python/catch_conflicts/catch_conflicts_py2.py

···
+import pkg_resources
+import collections
+import sys
+
+do_abort = False
+packages = collections.defaultdict(list)
+
+for f in sys.path:
+    for req in pkg_resources.find_distributions(f):
+        if req not in packages[req.project_name]:
+            # some exceptions inside buildPythonPackage
+            if req.project_name in ['setuptools', 'pip', 'wheel']:
+                continue
+            packages[req.project_name].append(req)
+
+
+for name, duplicates in packages.items():
+    if len(duplicates) > 1:
+        do_abort = True
+        print("Found duplicated packages in closure for dependency '{}': ".format(name))
+        for dup in duplicates:
+            print("  " + repr(dup))
+
+if do_abort:
+    print("")
+    print(
+        'Package duplicates found in closure, see above. Usually this '
+        'happens if two packages depend on different version '
+        'of the same dependency.')
+    sys.exit(1)

pkgs/development/interpreters/python/hooks/default.nix

···
   pythonCatchConflictsHook = callPackage ({ makePythonHook, setuptools }:
     makePythonHook {
       name = "python-catch-conflicts-hook";
-      substitutions = {
+      substitutions = let
+        useLegacyHook = lib.versionOlder python.version "3.10";
+      in {
         inherit pythonInterpreter pythonSitePackages;
-        catchConflicts=../catch_conflicts/catch_conflicts.py;
+        catchConflicts = if useLegacyHook then
+          ../catch_conflicts/catch_conflicts_py2.py
+        else
+          ../catch_conflicts/catch_conflicts.py;
+      } // lib.optionalAttrs useLegacyHook {
+        inherit setuptools;
       };
     } ./python-catch-conflicts-hook.sh) {};