Fix type to Py_BuildValue

Author: benjeffery

python/CHANGELOG.rst

@@ -15,6 +15,11 @@
 - Add ``keep_rows`` method to table classes to support efficient in-place
   table subsetting (:user:`jeromekelleher`, :pr:`2700`)
 
+**Bugfixes**
+
+- Fix `UnicodeDecodeError` when calling `Variant.alleles` on the `emscripten` platform.
+  (:user:`benjeffery`, :pr:`2754`, :issue:`2737`)
+
 --------------------
 [0.5.4] - 2023-01-13
 --------------------

python/_tskitmodule.c

@@ -578,7 +578,8 @@ make_alleles(tsk_variant_t *variant)
         goto out;
     }
     for (j = 0; j < variant->num_alleles; j++) {
-        item = Py_BuildValue("s#", variant->alleles[j], variant->allele_lengths[j]);
+        item = Py_BuildValue(
+            "s#", variant->alleles[j], (Py_ssize_t) variant->allele_lengths[j]);
         if (item == NULL) {
             Py_DECREF(t);
             goto out;