Bump Python requirement drf-spectacular to 0.21.0

requirements.txt

@@ -9,7 +9,7 @@ django-enumfields==2.1.1
 django-redis==5.0.0
 django-rq==2.4.1
 djangorestframework==3.12.4
-drf-spectacular==0.18.2
+drf-spectacular==0.21.0
 gitpython==3.1.24
 python-gitlab==2.10.1
 python-memcached==1.59