bpo-44676: Serialize the union type using only public API (GH-27323) (GH-27340)

Remove also the _from_args() constructor.
(cherry picked from commit 435a0334d341e5f8faed594d9f015746bb7845db)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
diff --git a/Lib/typing.py b/Lib/typing.py
index 0570b4e..ae31589 100644
--- a/Lib/typing.py
+++ b/Lib/typing.py
@@ -321,7 +321,7 @@ def _eval_type(t, globalns, localns, recursive_guard=frozenset()):
         if isinstance(t, GenericAlias):
             return GenericAlias(t.__origin__, ev_args)
         if isinstance(t, types.Union):
-            return types.Union._from_args(ev_args)
+            return functools.reduce(operator.or_, ev_args)
         else:
             return t.copy_with(ev_args)
     return t
@@ -1806,7 +1806,7 @@ def _strip_annotations(t):
         stripped_args = tuple(_strip_annotations(a) for a in t.__args__)
         if stripped_args == t.__args__:
             return t
-        return types.Union._from_args(stripped_args)
+        return functools.reduce(operator.or_, stripped_args)
 
     return t