Avoid interactive pdb when pytest tests itself - fix #2023

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

_pytest/pytester.py

@@ -916,8 +916,11 @@ class Testdir:
         env['PYTHONPATH'] = os.pathsep.join(filter(None, [
             str(os.getcwd()), env.get('PYTHONPATH', '')]))
         kw['env'] = env
-        return subprocess.Popen(cmdargs,
-                                stdout=stdout, stderr=stderr, **kw)
+
+        popen = subprocess.Popen(cmdargs, stdin=subprocess.PIPE, stdout=stdout, stderr=stderr, **kw)
+        popen.stdin.close()
+
+        return popen
 
     def run(self, *cmdargs):
         """Run a command with arguments.

changelog/2023.bugfix

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