vendor asynccontextmanager outside of greenlet dependency

Fixed installation issue where the ``sqlalchemy.dialects.mysql`` module
would not be importable if "greenlet" were not installed.

This was actually breaking the sphinx build for when greenlet
were not installed.

Fixes: #7204
Change-Id: Ia351c124a2f1ca44bafe20a97267ce20cb55808f

1 files changed, 10 insertions, 4 deletions

diff --git a/lib/sqlalchemy/util/concurrency.py b/lib/sqlalchemy/util/concurrency.py
index 5d15e06de..37ecfdbc3 100644
--- a/lib/sqlalchemy/util/concurrency.py
+++ b/lib/sqlalchemy/util/concurrency.py
@@ -1,3 +1,10 @@
+# util/concurrency.py
+# Copyright (C) 2005-2021 the SQLAlchemy authors and contributors
+# <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: https://www.opensource.org/licenses/mit-license.php
+
 from . import compat
 
 have_greenlet = False
@@ -19,7 +26,9 @@ if compat.py3k:
             _util_async_run_coroutine_function,
         )  # noqa F401, E501
         from ._concurrency_py3k import asyncio  # noqa F401
-        from ._concurrency_py3k import asynccontextmanager
+
+    # does not need greennlet, just Python 3
+    from ._compat_py3k import asynccontextmanager  # noqa F401
 
 if not have_greenlet:
 
@@ -58,6 +67,3 @@ if not have_greenlet:
 
     def _util_async_run_coroutine_function(fn, *arg, **kw):  # noqa F81
         _not_implemented()
-
-    def asynccontextmanager(fn, *arg, **kw):  # noqa F81
-        _not_implemented()