Fix duration field inspector

- Durations should allow min and max to be set
- Durations should be represented as an `openapi.TYPE_STRING` so swagger
  and other tools do not prevent valid input ("00:10:00" is valid, but
  not a number)

See:

https://www.django-rest-framework.org/api-guide/fields/#durationfield and
https://docs.djangoproject.com/en/3.0/ref/utils/#django.utils.dateparse.parse_duration

src/drf_yasg/inspectors/field.py

@@ -326,11 +326,16 @@ def find_regex(regex_field):
     return pattern
 
 
-numeric_fields = (serializers.IntegerField, serializers.FloatField, serializers.DecimalField)
+comparable_fields = (
+    serializers.DecimalField,
+    serializers.DurationField,
+    serializers.FloatField,
+    serializers.IntegerField,
+)
 limit_validators = [
     # minimum and maximum apply to numbers
-    (validators.MinValueValidator, numeric_fields, 'minimum', operator.__gt__),
-    (validators.MaxValueValidator, numeric_fields, 'maximum', operator.__lt__),
+    (validators.MinValueValidator, comparable_fields, 'minimum', operator.__gt__),
+    (validators.MaxValueValidator, comparable_fields, 'maximum', operator.__lt__),
 
     # minLength and maxLength apply to strings
     (validators.MinLengthValidator, serializers.CharField, 'min_length', operator.__gt__),
@@ -391,7 +396,7 @@ def decimal_field_type(field):
     (models.DateTimeField, (openapi.TYPE_STRING, openapi.FORMAT_DATETIME)),
     (models.DateField, (openapi.TYPE_STRING, openapi.FORMAT_DATE)),
     (models.DecimalField, (decimal_field_type, openapi.FORMAT_DECIMAL)),
-    (models.DurationField, (openapi.TYPE_INTEGER, None)),
+    (models.DurationField, (openapi.TYPE_STRING, None)),
     (models.FloatField, (openapi.TYPE_NUMBER, None)),
     (models.IntegerField, (openapi.TYPE_INTEGER, None)),
     (models.IPAddressField, (openapi.TYPE_STRING, openapi.FORMAT_IPV4)),
@@ -418,7 +423,7 @@ def decimal_field_type(field):
     (serializers.IntegerField, (openapi.TYPE_INTEGER, None)),
     (serializers.FloatField, (openapi.TYPE_NUMBER, None)),
     (serializers.DecimalField, (decimal_field_type, openapi.FORMAT_DECIMAL)),
-    (serializers.DurationField, (openapi.TYPE_NUMBER, None)),  # ?
+    (serializers.DurationField, (openapi.TYPE_STRING, None)),
     (serializers.DateField, (openapi.TYPE_STRING, openapi.FORMAT_DATE)),
     (serializers.DateTimeField, (openapi.TYPE_STRING, openapi.FORMAT_DATETIME)),
     (serializers.ModelField, (openapi.TYPE_STRING, None)),