Convert to np.datetime64 directly instead of using np.asarray

Author: weiji14

pygmt/clib/session.py

@@ -720,7 +720,7 @@ def _check_dtype_and_dim(self, array, ndim):
         if array.dtype.type not in DTYPES:
             try:
                 # Try to convert any unknown numpy data types to np.datetime64
-                array = np.asarray(array, dtype=np.datetime64)
+                array = np.datetime64(array)
             except ValueError:
                 raise GMTInvalidInput(
                     "Unsupported numpy data type '{}'.".format(array.dtype.type)