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

`_RandomNameSequence` is not true singleton so using `os.register_at_fork` doesn't make sense unlike `random._inst`.

This reverts commit 8e409cebad.

Lib/tempfile.py

@@ -129,22 +129,24 @@ class _RandomNameSequence:
 
     _RandomNameSequence is an iterator."""
 
-    def __init__(self, characters="abcdefghijklmnopqrstuvwxyz0123456789_", length=8, rng=None):
-        if rng is None:
-            rng = _Random()
-        if hasattr(_os, "fork"):
-            # prevent same state after fork
-            _os.register_at_fork(after_in_child=rng.seed)
-        self.rng = rng
-        self.characters = characters
-        self.length = length
+    characters = "abcdefghijklmnopqrstuvwxyz0123456789_"
+
+    @property
+    def rng(self):
+        cur_pid = _os.getpid()
+        if cur_pid != getattr(self, '_rng_pid', None):
+            self._rng = _Random()
+            self._rng_pid = cur_pid
+        return self._rng
 
     def __iter__(self):
         return self
 
     def __next__(self):
         c = self.characters
-        return ''.join(self.rng.choices(c, k=self.length))
+        choose = self.rng.choice
+        letters = [choose(c) for dummy in range(8)]
+        return ''.join(letters)
 
 def _candidate_tempdir_list():
     """Generate a list of candidate temporary directories which