Use multiprocessing start_method "forkserver"

Alternative to PR pybind#4305

Author: rwgk

tests/conftest.py

@@ -7,6 +7,8 @@
 import contextlib
 import difflib
 import gc
+import multiprocessing
+import os
 import re
 import textwrap
 
@@ -15,6 +17,9 @@
 # Early diagnostic for failed imports
 import pybind11_tests
 
+if os.name != "nt":
+    multiprocessing.set_start_method("forkserver")
+
 _long_marker = re.compile(r"([0-9])L")
 _hexadecimal = re.compile(r"0x[0-9a-fA-F]+")
 

tests/test_gil_scoped.py

@@ -144,7 +144,7 @@ def _intentional_deadlock():
 
 
 ALL_BASIC_TESTS_PLUS_INTENTIONAL_DEADLOCK = ALL_BASIC_TESTS + (_intentional_deadlock,)
-SKIP_IF_DEADLOCK = True  # See PR #4216
+SKIP_IF_DEADLOCK = False  # See PR #4216
 
 
 def _run_in_process(target, *args, **kwargs):