Update minimum Pillow requirement to 7.0.0

- Includes fix for security issue CVE-2019-16865 before 6.2.0
- Default rescale algorithm is now bilinear, or nearest for palette
  images. No need to pass lanczos on our own.

src/backend/matrix_client.py

@@ -889,7 +889,7 @@ class MatrixClient(nio.AsyncClient):
             raise UneededThumbnail()
 
         if not small:
-            thumb.thumbnail((800, 600), PILImage.LANCZOS)
+            thumb.thumbnail((800, 600))
 
         with io.BytesIO() as out:
             if thumb.mode in png_modes: