return None in numpy_letter_types_to_numba_types on unsupported types
return None in numpy_letter_types_to_numba_types on unsupported types

more convenient than raising an error.
stefanseefeld/numba,jriehl/numba,gdementen/numba,jriehl/numba,cpcloud/numba,numba/numba,pombredanne/numba,gmarkall/numba,jriehl/numba,jriehl/numba,stuartarchibald/numba,IntelLabs/numba,IntelLabs/numba,stuartarchibald/numba,stuartarchibald/numba,pombredanne/numba,IntelLabs/numba,seibert/numba,pitrou/numba,seibert/numba,sklam/numba,stefanseefeld/numba,cpcloud/numba,sklam/numba,ssarangi/numba,ssarangi/numba,pombredanne/numba,numba/numba,GaZ3ll3/numba,jriehl/numba,gdementen/numba,seibert/numba,IntelLabs/numba,gmarkall/numba,gmarkall/numba,stefanseefeld/numba,pitrou/numba,pombredanne/numba,gmarkall/numba,gmarkall/numba,GaZ3ll3/numba,pitrou/numba,gdementen/numba,sklam/numba,IntelLabs/numba,ssarangi/numba,numba/numba,stonebig/numba,sklam/numba,GaZ3ll3/numba,stuartarchibald/numba,pitrou/numba,stonebig/numba,ssarangi/numba,seibert/numba,stonebig/numba,pitrou/numba,ssarangi/numba,numba/numba,stefanseefeld/numba,numba/numba,gdementen/numba,stuartarchibald/numba,cpcloud/numba,GaZ3ll3/numba,pombredanne/numba,stefanseefeld/numba,seibert/numba,stonebig/numba,sklam/numba,gdementen/numba,stonebig/numba,cpcloud/numba,GaZ3ll3/numba,cpcloud/numba