utils.misc: Make nullcontext work with asyncio

Implement __aenter__ and __aexit__ on nullcontext so it can be used as
an asynchronous context manager.

devlib/utils/misc.py

@@ -748,8 +748,7 @@ def batch_contextmanager(f, kwargs_list):
         yield
 
 
-@contextmanager
-def nullcontext(enter_result=None):
+class nullcontext:
     """
     Backport of Python 3.7 ``contextlib.nullcontext``
 
@@ -761,7 +760,20 @@ def nullcontext(enter_result=None):
         statement, or `None` if nothing is specified.
     :type enter_result: object
     """
-    yield enter_result
+    def __init__(self, enter_result=None):
+        self.enter_result = enter_result
+
+    def __enter__(self):
+        return self.enter_result
+
+    async def __aenter__(self):
+        return self.enter_result
+
+    def __exit__(*_):
+        return
+
+    async def __aexit__(*_):
+        return
 
 
 class tls_property: