Allow importing numba-dpex even if dpctl shows that there are no SYCL platforms

[PokhodenkoSA]

Currently import numba-dppy raises exception if SYCL platform is not available.
https://github.com/IntelPython/numba-dppy/blob/6033fb832444e8e9fb5a0e12d300eaf4fdf438f9/numba_dppy/__init__.py#L527

It uses dpctl:
https://github.com/IntelPython/numba-dppy/blob/6033fb832444e8e9fb5a0e12d300eaf4fdf438f9/numba_dppy/config.py#L21

It means that if SYCL is not available we can not import numba-dppy and its tests at all.

Why not we able to import numba-dppy and run tests which do not use SYCL devices? We could skip all SYCL dependent tests but others could be run.

@diptorupd @reazulhoque
What do you think? Is it not too strict to prohibit import numba-dppy?

It is related to #38. It is confusing when installed package behaves like it is not installed.