strings2arrays: make work again for sequences of inequal length

PR explosion#897 fixed the dtypes in strings2arrays, however also broke
strings2arrays for batches with sequences if inequal lengths.

thinc/layers/strings2arrays.py

@@ -1,4 +1,5 @@
 from typing import Callable, List, Sequence, Tuple
+from ctypes import c_uint64
 
 from murmurhash import hash_unicode
 
@@ -17,8 +18,10 @@ def strings2arrays() -> Model[InT, OutT]:
 
 
 def forward(model: Model[InT, OutT], Xs: InT, is_train: bool) -> Tuple[OutT, Callable]:
-    hashes = [[hash_unicode(word) for word in X] for X in Xs]
-    hash_arrays = [model.ops.asarray2i(h, dtype="uint64") for h in hashes]
+    # Cast 32-bit (signed) integer to 64-bit unsigned, since such casting
+    # is deprecated in NumPy.
+    hashes = [[c_uint64(hash_unicode(word)).value for word in X] for X in Xs]
+    hash_arrays = [model.ops.asarray1i(h, dtype="uint64") for h in hashes]
     arrays = [model.ops.reshape2i(array, -1, 1) for array in hash_arrays]
 
     def backprop(dX: OutT) -> InT: