greg.api.filters: use explicit symbol imports

Mostly a visual improvement, but instead of importing the entire
django_filters.rest_framework module we make the dependency situation
clearer by importing specific symbols.

Fixes: GREG-20

greg/api/filters.py

-from django_filters import rest_framework as filters
+from django_filters.rest_framework import (
+    BaseInFilter,
+    BooleanFilter,
+    FilterSet,
+)
 
-from greg.models import Person, PersonRole, PersonIdentity
+from greg.models import (
+    Person,
+    PersonRole,
+    PersonIdentity,
+)
 
 
-class PersonRoleFilter(filters.FilterSet):
-    type = filters.BaseInFilter(field_name="role__type", lookup_expr="in")
+class PersonRoleFilter(FilterSet):
+    type = BaseInFilter(field_name="role__type", lookup_expr="in")
 
     class Meta:
         model = PersonRole
         fields = ["type"]
 
 
-class PersonFilter(filters.FilterSet):
-    verified = filters.BooleanFilter(
+class PersonFilter(FilterSet):
+    verified = BooleanFilter(
         field_name="person__verified_by_id", lookup_expr="isnull", exclude=True
     )
 
@@ -21,7 +29,7 @@ class PersonFilter(filters.FilterSet):
         fields = ["first_name", "last_name", "verified"]
 
 
-class PersonIdentityFilter(filters.FilterSet):
+class PersonIdentityFilter(FilterSet):
     class Meta:
         model = PersonIdentity
         fields = ["type", "verified_by_id"]