Remove --boxed and pytest-forked dependency

Fixes #468
Fixes #543

testing/acceptance_test.py

@@ -602,11 +602,10 @@ def test_config_initialization(
 def test_crashing_item(pytester, when) -> None:
     """Ensure crashing item is correctly reported during all testing stages"""
     code = dict(setup="", call="", teardown="")
-    code[when] = "py.process.kill(os.getpid())"
+    code[when] = "os._exit(1)"
     p = pytester.makepyfile(
         """
         import os
-        import py
         import pytest
 
         @pytest.fixture

testing/test_plugin.py

@@ -122,22 +122,6 @@ def test_hook_auto_num_workers(
     assert config.getoption("numprocesses") == 42
 
 
-def test_boxed_with_collect_only(pytester: pytest.Pytester) -> None:
-    from xdist.plugin import pytest_cmdline_main as check_options
-
-    config = pytester.parseconfigure("-n1", "--boxed")
-    check_options(config)
-    assert config.option.forked
-
-    config = pytester.parseconfigure("-n1", "--collect-only")
-    check_options(config)
-    assert not config.option.forked
-
-    config = pytester.parseconfigure("-n1", "--boxed", "--collect-only")
-    check_options(config)
-    assert config.option.forked
-
-
 def test_dsession_with_collect_only(pytester: pytest.Pytester) -> None:
     from xdist.plugin import pytest_cmdline_main as check_options
     from xdist.plugin import pytest_configure as configure