Fix warning in openslide test (#1009)

```
ResourceWarning: unclosed file <_io.BufferedReader name='/input/tests/data/test.tif'>
  testMethod()
ResourceWarning: Enable tracemalloc to get the object allocation traceback
```

tests/test_openslide.py

@@ -4,6 +4,5 @@
 
 class TestOpenslide(unittest.TestCase):
  def test_read_tif(self):
- slide = open_slide('/input/tests/data/test.tif')
-
- self.assertEqual(1, slide.level_count)
+ with open_slide('/input/tests/data/test.tif') as slide:
+ self.assertEqual(1, slide.level_count)