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
6 jobs for fix-deprecation-warning-abc in 2 minutes and 16 seconds (queued for 3 seconds)
Name Stage Failure
failed
minimal-ubuntu Test
    from pystencils.gpucuda.indexing import indexing_creator_from_params
File "/builds/seitz/pystencils/pystencils/gpucuda/__init__.py", line 1, in <module>
from pystencils.gpucuda.kernelcreation import create_cuda_kernel, created_indexed_cuda_kernel
File "/builds/seitz/pystencils/pystencils/gpucuda/kernelcreation.py", line 3, in <module>
from pystencils.gpucuda.indexing import BlockIndexing
File "/builds/seitz/pystencils/pystencils/gpucuda/indexing.py", line 18, in <module>
class AbstractIndexing(abc.ABC):
AttributeError: module 'collections.abc' has no attribute 'ABC'
ERROR: Job failed: exit code 1
failed
build-documentation Test
  File "/builds/seitz/pystencils/pystencils/gpucuda/__init__.py", line 1, in <module>
from pystencils.gpucuda.kernelcreation import create_cuda_kernel, created_indexed_cuda_kernel
File "/builds/seitz/pystencils/pystencils/gpucuda/kernelcreation.py", line 3, in <module>
from pystencils.gpucuda.indexing import BlockIndexing
File "/builds/seitz/pystencils/pystencils/gpucuda/indexing.py", line 18, in <module>
class AbstractIndexing(abc.ABC):
AttributeError: module 'collections.abc' has no attribute 'ABC'

ERROR: Job failed: exit code 1
failed
minimal-conda Test
    from pystencils.gpucuda.indexing import indexing_creator_from_params
File "/builds/seitz/pystencils/pystencils/gpucuda/__init__.py", line 1, in <module>
from pystencils.gpucuda.kernelcreation import create_cuda_kernel, created_indexed_cuda_kernel
File "/builds/seitz/pystencils/pystencils/gpucuda/kernelcreation.py", line 3, in <module>
from pystencils.gpucuda.indexing import BlockIndexing
File "/builds/seitz/pystencils/pystencils/gpucuda/indexing.py", line 18, in <module>
class AbstractIndexing(abc.ABC):
AttributeError: module 'collections.abc' has no attribute 'ABC'
ERROR: Job failed: exit code 1
failed
minimal-windows Test
    from pystencils.gpucuda.indexing import indexing_creator_from_params
File "C:\cygwin64\home\build\builds\seitz\pystencils\pystencils\gpucuda\__init__.py", line 1, in <module>
from pystencils.gpucuda.kernelcreation import create_cuda_kernel, created_indexed_cuda_kernel
File "C:\cygwin64\home\build\builds\seitz\pystencils\pystencils\gpucuda\kernelcreation.py", line 3, in <module>
from pystencils.gpucuda.indexing import BlockIndexing
File "C:\cygwin64\home\build\builds\seitz\pystencils\pystencils\gpucuda\indexing.py", line 18, in <module>
class AbstractIndexing(abc.ABC):
AttributeError: module 'collections.abc' has no attribute 'ABC'
ERROR: Job failed: Process exited with: 1. Reason was: ()
failed
tests-and-coverage Test
pystencils/gpucuda/kernelcreation.py:3: in <module>
from pystencils.gpucuda.indexing import BlockIndexing
pystencils/gpucuda/indexing.py:18: in <module>
class AbstractIndexing(abc.ABC):
E AttributeError: module 'collections.abc' has no attribute 'ABC'
Uploading artifacts...
WARNING: coverage_report: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1