fix build warning: use of numpy import_array()

src/pnetcdf/_utils.c: In function '__pyx_pymod_exec__utils':
src/pnetcdf/_utils.c:17646:3: warning: returning 'void *' from a function with return type 'int' makes integer from pointer without a cast [-Wint-conversion]
   import_array();
   ^~~~~~~~~~~~

src/pnetcdf/_Variable.c: In function '__pyx_pymod_exec__Variable':
src/pnetcdf/_Variable.c:77507:3: warning: returning 'void *' from a function with return type 'int' makes integer from pointer without a cast [-Wint-conversion]
   import_array();
   ^~~~~~~~~~~~

Author: wkliao

src/pnetcdf/_Variable.pyx

@@ -23,7 +23,9 @@ from ._utils cimport _strencode, _check_err, _set_att, _get_att, _get_att_names,
 from ._utils import chartostring
 from ._utils cimport _nptonctype, _notcdf2dtypes, _nctonptype, _nptompitype, _supportedtypes, _supportedtypescdf2, \
                      default_fillvals, _StartCountStride, _out_array_shape, _private_atts
-import_array()
+
+cimport numpy
+numpy.import_array()
 
 
 ctypedef MPI.Datatype Datatype

src/pnetcdf/_utils.pyx

@@ -28,7 +28,8 @@ MPI_Datatype MPI_UNSIGNED64 = MPI.UINT64
 MPI_Datatype MPI_FLOAT = MPI.FLOAT
 MPI_Datatype MPI_DOUBLE = MPI.DOUBLE"""
 
-import_array()
+cimport numpy
+numpy.import_array()
 
 # np data type <--> netCDF data type mapping.
 _nptonctype  = {'S1' : NC_CHAR_C,