bpo-36373: Deprecate explicit loop in task and subprocess API (GH-16033)

diff --git a/Lib/asyncio/subprocess.py b/Lib/asyncio/subprocess.py
index e4f9e52..bddfb01 100644
--- a/Lib/asyncio/subprocess.py
+++ b/Lib/asyncio/subprocess.py
@@ -224,6 +224,13 @@
                                   **kwds):
     if loop is None:
         loop = events.get_event_loop()
+    else:
+        warnings.warn("The loop argument is deprecated since Python 3.8 "
+                      "and scheduled for removal in Python 3.10.",
+                      DeprecationWarning,
+                      stacklevel=2
+        )
+
     protocol_factory = lambda: SubprocessStreamProtocol(limit=limit,
                                                         loop=loop,
                                                         _asyncio_internal=True)
@@ -239,6 +246,12 @@
                                  limit=streams._DEFAULT_LIMIT, **kwds):
     if loop is None:
         loop = events.get_event_loop()
+    else:
+        warnings.warn("The loop argument is deprecated since Python 3.8 "
+                      "and scheduled for removal in Python 3.10.",
+                      DeprecationWarning,
+                      stacklevel=2
+        )
     protocol_factory = lambda: SubprocessStreamProtocol(limit=limit,
                                                         loop=loop,
                                                         _asyncio_internal=True)