Avoid interactive pdb when pytest tests itself - fix pytest-dev#2023

The debugging.py calls post_mortem() on error and pdb will drops an
interactive debugger when the stdin is a readable fd.

AUTHORS

@@ -9,6 +9,7 @@ Ahn Ki-Wook
 Alexander Johnson
 Alexei Kozlenok
 Anatoly Bubenkoff
+Andras Tim
 Andreas Zeidler
 Andrzej Ostrowski
 Andy Freeland

_pytest/pytester.py

@@ -909,7 +909,7 @@ def popen(self, cmdargs, stdout, stderr, **kw):
             str(os.getcwd()), env.get('PYTHONPATH', '')]))
         kw['env'] = env
         return subprocess.Popen(cmdargs,
-                                stdout=stdout, stderr=stderr, **kw)
+                                stdin=subprocess.DEVNULL, stdout=stdout, stderr=stderr, **kw)
 
     def run(self, *cmdargs):
         """Run a command with arguments.

changelog/2023.bugfix

@@ -0,0 +1 @@
+Set ``stdin`` to ``DEVNULL`` in ``pytester.py.Testdir.popen()`` for avoid unwanted interactive ``pdb``