fix(asyncio): accept unsized iterables in as_completed

Since Python 3.12 asyncio.as_completed accepts a generator of awaitables,
but the tqdm wrapper calls len(fs) unconditionally when total is omitted,
so the same input raises TypeError: object of type 'generator' has no len().

Fall back to an unknown total when fs has no length, mirroring how
tqdm.std.tqdm.__init__ derives total from an iterable. Sized inputs keep
reporting their exact total, and an explicit total is still honoured.

Fixes #1811

tests/tests_asyncio.py

@@ -120,6 +120,26 @@ async def test_as_completed(capsys, tol):
                 raise
 
 
+@mark.asyncio
+async def test_as_completed_unsized(capsys):
+    """Test asyncio as_completed with an unsized iterable"""
+    fs = (asyncio.sleep(0, result=i) for i in range(3))
+    res = sorted([await f for f in as_completed(fs)])
+    assert res == [0, 1, 2]
+    _, err = capsys.readouterr()
+    assert '3it' in err
+
+
+@mark.asyncio
+async def test_as_completed_unsized_with_total(capsys):
+    """Test asyncio as_completed with an unsized iterable and an explicit total"""
+    fs = (asyncio.sleep(0, result=i) for i in range(3))
+    res = sorted([await f for f in as_completed(fs, total=3)])
+    assert res == [0, 1, 2]
+    _, err = capsys.readouterr()
+    assert '3/3' in err
+
+
 async def double(i):
     return i * 2