Bump pillow from 7.1.2 to 8.1.1

Bumps [pillow](https://github.com/python-pillow/Pillow) from 7.1.2 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/7.1.2...8.1.1)

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

requirements.txt

@@ -1,5 +1,5 @@
 idna==2.8
 numpy==1.18.1
-Pillow==7.1.2
+Pillow==8.1.1
 tweepy==3.8.0
 urllib3==1.25.8