fixed imports, jit, and backend tests

src/pystencils/backend/jit/__init__.py

src/pystencils/backend/jit/cpu_extension_module.py

src/pystencils/backend/jit/jit.py

src/pystencils/backend/jit/legacy_cpu.py

src/pystencils/backend/jit/msvc_detection.py

src/pystencils/backend/kernelcreation/__init__.py

src/pystencils/backend/kernelcreation/analysis.py

src/pystencils/datahandling/parallel_datahandling.py

src/pystencils/kernelcreation.py

src/pystencils/old/cpu/__init__.py

src/pystencils/old/gpu/__init__.py

tests/nbackend/kernelcreation/platform/test_basic_cpu.py

tests/nbackend/kernelcreation/test_domain_kernels.py

tests/nbackend/kernelcreation/test_freeze.py

tests/nbackend/kernelcreation/test_iteration_space.py

tests/nbackend/kernelcreation/test_options.py

tests/nbackend/kernelcreation/test_typification.py

tests/nbackend/test_constant_folding.py

tests/nbackend/test_cpujit.py

tests/nbackend/test_expressions.py