bpo-42160: tempfile: Reduce overhead of pid check. (GH-22997)

The _RandomSequence class in tempfile used to check the current pid every time its rng property was used.
This commit replaces this code with `os.register_at_fork` to reduce the overhead.
diff --git a/Lib/test/test_tempfile.py b/Lib/test/test_tempfile.py
index 8ace883..77d710e 100644
--- a/Lib/test/test_tempfile.py
+++ b/Lib/test/test_tempfile.py
@@ -153,8 +153,8 @@ def setUp(self):
         self.r = tempfile._RandomNameSequence()
         super().setUp()
 
-    def test_get_six_char_str(self):
-        # _RandomNameSequence returns a six-character string
+    def test_get_eight_char_str(self):
+        # _RandomNameSequence returns a eight-character string
         s = next(self.r)
         self.nameCheck(s, '', '', '')