Mask top level import of `.hilevel`

Since it isn't required until the landing of the new service-manager
stuff in #12; was an oversight
from commit `0607a31dddeba032a2cf7d9fe605edd9d7bb4846`.

tractor/__init__.py

@@ -67,4 +67,4 @@ from ._root import (
 from ._ipc import Channel as Channel
 from ._portal import Portal as Portal
 from ._runtime import Actor as Actor
-from . import hilevel as hilevel
+# from . import hilevel as hilevel

tractor/_context.py

@@ -47,6 +47,9 @@ from functools import partial
 import inspect
 from pprint import pformat
 import textwrap
+from types import (
+    UnionType,
+)
 from typing import (
     Any,
     AsyncGenerator,
@@ -2548,7 +2551,14 @@ def context(
     name: str
     param: Type
     for name, param in annots.items():
-        if param is Context:
+        if (
+            param is Context
+            or (
+                isinstance(param, UnionType)
+                and
+                Context in param.__args__
+            )
+        ):
             ctx_var_name: str = name
             break
     else: