Callable passed to Min/Max validators breaks in the fields

[Harro]

in #29860 for django 2.2 the option was added to pass in a callable.

I just upgraded to django 2.2 and found our custom max value validator for current year + 1 (which gave us a migration on the year change) and thought to replace it with the new mechanic.

The result was the following error:
`

File "<venv>/lib/python3.6/site-packages/django/db/models/fields/init.py", line 1799, in <genexpr>

validator.limit_value <= max_value for validator in validators_)):

`

So the field wants to check that the validator's max (and min) value lie within range of the Integer field which is not possible with the callable.

The fix should be simple: Check if it's a callable and call it for the value.