pytester: workaround issue causing spawn to crash or hang

In pytester tests, pytest stashes & restores the sys.modules for each
test. So if the test imports a new module, it is initialized anew each
time.

Turns out the readline module isn't multi-init safe, which causes
pytester.spawn to crash or hang. So preserve it as a workaround.

changelog/7913.bugfix.rst

@@ -0,0 +1 @@
+Fixed a crash or hang in ``pytester.spawn`` when the ``readline`` module is involved.

src/_pytest/pytester.py

@@ -707,8 +707,11 @@ class Pytester:
         # Some zope modules used by twisted-related tests keep internal state
         # and can't be deleted; we had some trouble in the past with
         # `zope.interface` for example.
+        #
+        # Preserve readline due to https://bugs.python.org/issue41033.
+        # pexpect issues a SIGWINCH.
         def preserve_module(name):
-            return name.startswith("zope")
+            return name.startswith(("zope", "readline"))
 
         return SysModulesSnapshot(preserve=preserve_module)