Improve rounding

Tests/test_image_thumbnail.py

@@ -58,6 +58,10 @@ def test_aspect():
     im.thumbnail((50, 50))
     assert im.size == (34, 50)  # ratio is 0.68
 
+    im = Image.new("L", (100, 30))  # ratio is 3.333333333333
+    im.thumbnail((75, 75))
+    assert im.size == (75, 23)  # ratio is 3.260869565217
+
 
 def test_float():
     im = Image.new("L", (128, 128))

src/PIL/Image.py

@@ -2240,13 +2240,20 @@ def thumbnail(self, size, resample=BICUBIC, reducing_gap=2.0):
         if x >= self.width and y >= self.height:
             return
 
+        def round_aspect(number):
+            if x / y >= aspect:
+                key = lambda n: abs(aspect - n / y)  # noqa: E731
+            else:
+                key = lambda n: abs(aspect - x / n)  # noqa: E731
+            return max(min(math.floor(number), math.ceil(number), key=key), 1)
+
         # preserve aspect ratio
         aspect = self.width / self.height
         if x / y >= aspect:
-            x = max(y * aspect, 1)
+            x = round_aspect(y * aspect)
         else:
-            y = max(x / aspect, 1)
-        size = (round(x), round(y))
+            y = round_aspect(x / aspect)
+        size = (x, y)
 
         box = None
         if reducing_gap is not None: