Simplify INPUTRC insertion

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: johnslavik

Lib/test/support/pty_helper.py

@@ -22,9 +22,8 @@ def run_pty(script, input=b"dummy input\r", env=None, readline=None):
         # Isolate readline from personal init files by setting INPUTRC
         # to an empty file. See also GH-142353.
         if env is None:
-            env = {**os.environ.copy(), "INPUTRC": os.devnull}
-        else:
-            env.setdefault("INPUTRC", os.devnull)
+            env = os.environ.copy()
+        env.setdefault("INPUTRC", os.devnull)
 
     proc = subprocess.Popen(args, stdin=slave, stdout=slave, stderr=slave, env=env)
     os.close(slave)