Drop useless fork error branch

tests/conftest.py

@@ -52,7 +52,7 @@ def arb_addr():
 
 
 def pytest_generate_tests(metafunc):
-    spawn_backend = metafunc.config.getoption("spawn_backend")
+    spawn_backend = metafunc.config.option.spawn_backend
     assert spawn_backend in ('mp', 'trio_run_in_process')
 
     if 'start_method' in metafunc.fixturenames:

tractor/_spawn.py

@@ -72,11 +72,6 @@ def try_set_start_method(name: str) -> Optional[mp.context.BaseContext]:
         raise ValueError(
             f"Spawn method `{name}` is invalid please choose one of {methods}"
         )
-
-    elif name == 'fork':
-        raise ValueError(
-            "`fork` is unsupported due to incompatibility with `trio`"
-        )
     elif name == 'forkserver':
         _forkserver_override.override_stdlib()
         _ctx = mp.get_context(name)