Refs #29942 -- Fixed docs build on Python < 3.9 avoiding dict union o…

…perator.

The docs build process was failing on djangoproject.com since it uses Python 3.8 and the dict union operator was added in Python 3.9.

docs/_ext/github_links.py

@@ -41,10 +41,10 @@ def visit_ImportFrom(self, node):
                 file = module_name_to_file_path(node.module)
                 file_contents = file.read_text(encoding="utf-8")
                 locator = CodeLocator.from_code(file_contents)
-                self.import_locations |= locator.import_locations
-                self.import_locations |= {
-                    n: node.module for n in locator.node_line_numbers if "." not in n
-                }
+                self.import_locations.update(locator.import_locations)
+                self.import_locations.update(
+                    {n: node.module for n in locator.node_line_numbers if "." not in n}
+                )
             else:
                 self.import_locations[alias.name] = ("." * node.level) + (
                     node.module or ""