fix: MappingProxyType is not a constant and should therefor use default_factory

Fixes execution on Python 3.11

Prevents the following error:
```
ImportError while loading conftest '/home/stephan/projects/pystencils/conftest.py'.
conftest.py:14: in <module>
    from pystencils.cpu import cpujit
pystencils/__init__.py:10: in <module>
    from .config import CreateKernelConfig
pystencils/config.py:19: in <module>
    @dataclass
/usr/lib/python3.11/dataclasses.py:1220: in dataclass
    return wrap(cls)
/usr/lib/python3.11/dataclasses.py:1210: in wrap
    return _process_class(cls, init, repr, eq, order, unsafe_hash,
/usr/lib/python3.11/dataclasses.py:958: in _process_class
    cls_fields.append(_get_field(cls, name, type, kw_only))
/usr/lib/python3.11/dataclasses.py:815: in _get_field
    raise ValueError(f'mutable default {type(f.default)} for field '
E   ValueError: mutable default <class 'mappingproxy'> for field gpu_indexing_params is not allowed: use default_factory

```

pystencils/config.py

@@ -82,7 +82,7 @@ class CreateKernelConfig:
     """
     Either 'block' or 'line' , or custom indexing class, see `pystencils.gpucuda.AbstractIndexing`
     """
-    gpu_indexing_params: MappingProxyType = field(default=MappingProxyType({}))
+    gpu_indexing_params: MappingProxyType = field(default_factory=lambda: MappingProxyType({}))
     """
     Dict with indexing parameters (constructor parameters of indexing class)
     e.g. for 'block' one can specify '{'block_size': (20, 20, 10) }'.
@@ -121,12 +121,12 @@ class CreateKernelConfig:
     allow_double_writes: bool = False
     """
     If True, don't check if every field is only written at a single location. This is required
-    for example for kernels that are compiled with loop step sizes > 1, that handle multiple 
+    for example for kernels that are compiled with loop step sizes > 1, that handle multiple
     cells at once. Use with care!
     """
     skip_independence_check: bool = False
     """
-    Don't check that loop iterations are independent. This is needed e.g. for 
+    Don't check that loop iterations are independent. This is needed e.g. for
     periodicity kernel, that access the field outside the iteration bounds. Use with care!
     """