Bump pillow from 7.1.0 to 8.1.1 in /modules/sync

Bumps [pillow](https://github.com/python-pillow/Pillow) from 7.1.0 to 8.1.1.
- [Release notes](https://github.com/python-pillow/Pillow/releases)
- [Changelog](https://github.com/python-pillow/Pillow/blob/master/CHANGES.rst)
- [Commits](https://github.com/python-pillow/Pillow/compare/7.1.0...8.1.1)

Signed-off-by: dependabot[bot] <support@github.com>

modules/sync/requirements.txt

@@ -1,7 +1,7 @@
 cssselect==0.9.2
 lxml==4.6.2
 Markdown==2.6.6
-Pillow==7.1.0
+Pillow==8.1.1
 pymongo==3.3.0
 PyPDF2==1.26.0
 python-frontmatter==0.3.1