Fix deprecation warning: `collections.abc` instead of `abc`

DeprecationWarning: Using or importing the ABCs from 'collections'
instead of from '' is deprecated, and in 3.8 it will stop working

pystencils/datahandling/datahandling_interface.py

 import numpy as np
-from abc import ABC, abstractmethod
+from collections.abc import ABC, abstractmethod
 from typing import Optional, Callable, Sequence, Iterable, Tuple, Dict, Union
 from pystencils.field import Field
 

pystencils/gpucuda/indexing.py

-import abc
+import collections.abc as abc
 from typing import Tuple  # noqa
 import sympy as sp
 from pystencils.astnodes import Conditional, Block