setuponly: remove printing out/err from capman

This was added in pytest-dev@1a5e530b9.
The test from there provides the same output with this fix, which makes
sense, since it suspends capturing, reads the output and prints it after
resuming capturing again.  Therefore it can just leave it in the capture
buffer in the first place.

Fixes pytest-dev#5906

changelog/5906.bugfix.rst

@@ -0,0 +1 @@
+Fix crash with ``KeyboardInterrupt`` during ``--setup-show``.

src/_pytest/setuponly.py

@@ -1,5 +1,3 @@
-import sys
-
 import pytest
 
 
@@ -51,7 +49,6 @@ def _show_fixture_action(fixturedef, msg):
     capman = config.pluginmanager.getplugin("capturemanager")
     if capman:
         capman.suspend_global_capture()
-        out, err = capman.read_global_capture()
 
     tw = config.get_terminal_writer()
     tw.line()
@@ -74,8 +71,6 @@ def _show_fixture_action(fixturedef, msg):
 
     if capman:
         capman.resume_global_capture()
-        sys.stdout.write(out)
-        sys.stderr.write(err)
 
 
 @pytest.hookimpl(tryfirst=True)

testing/python/setup_only.py

@@ -244,7 +244,12 @@ def test_capturing(two):
 
     result = testdir.runpytest("--setup-only", p)
     result.stdout.fnmatch_lines(
-        ["this should be captured", "this should also be captured"]
+        [
+            "*- Captured stdout setup -*",
+            "this should be captured",
+            "*- Captured stderr setup -*",
+            "this should also be captured",
+        ]
     )