Copy async_lru for Python 3.7 compatibility

tests/external/test_utils.py

@@ -0,0 +1,19 @@
+from functools import partial
+
+from great_ai.external.async_lru import unpartial
+
+
+def test_unpartial():
+    def foo():
+        pass
+
+    assert unpartial(foo) is foo
+
+    bar = partial(foo)
+
+    assert unpartial(bar) is foo
+
+    for _ in range(10):
+        bar = partial(bar)
+
+    assert unpartial(bar) is foo