fix: cvat/requirements/base.txt to reduce vulnerabilities (#2907)

The following vulnerabilities are fixed by pinning transitive dependencies:
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-1081494
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-1081501
- https://snyk.io/vuln/SNYK-PYTHON-PILLOW-1081502

Co-authored-by: snyk-bot <snyk-bot@snyk.io>

cvat/requirements/base.txt

@@ -6,7 +6,7 @@ django-cacheops==5.0.1
 django-compressor==2.4
 django-rq==2.3.2
 EasyProcess==0.3
-Pillow==7.2.0
+Pillow==8.1.1
 numpy==1.19.5
 python-ldap==3.3.1
 pytz==2020.1