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.

requirements.txt

@@ -1,4 +1,4 @@
-Pillow          >= 5.4.1, < 6
+Pillow          >= 7.0.0, < 8
 pymediainfo     >= 4.1,   < 5
 cairosvg        >= 2.4.2, < 3
 aiofiles        >= 0.4.0, < 0.5

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: