Copy async_lru for Python 3.7 compatibility

tests/external/test_cache_clear.py

@@ -0,0 +1,22 @@
+import asyncio
+
+import pytest
+from great_ai.external.async_lru import alru_cache
+
+pytestmark = pytest.mark.asyncio
+
+
+async def test_cache_clear(check_lru, loop):
+    @alru_cache()
+    async def coro(val):
+        return val
+
+    inputs = [1, 2, 3]
+    coros = [coro(v) for v in inputs]
+    ret = await asyncio.gather(*coros)
+    assert ret == inputs
+    check_lru(coro, hits=0, misses=3, cache=3, tasks=0)
+
+    coro.cache_clear()
+
+    check_lru(coro, hits=0, misses=0, cache=0, tasks=0)