Use utf-8 codec in premisrw

This avoids the following issue:

    > return ' '.join([str(x) for x in texts])
                       ^^^^^^
    E UnicodeEncodeError: 'ascii' codec can't encode characters in
      position 42-44: ordinal not in range(128)

Only fixed in Py2. Py3 codepath will need more work overall.

metsrw/plugins/premisrw/premis.py

@@ -693,9 +693,12 @@ def data_find_text(data, path):
     if isinstance(el, (list, tuple)):
         texts = [child for child in el[1:]
                  if not isinstance(child, (tuple, list, dict))]
-        if texts:
-            return ' '.join([str(x) for x in texts])
-    return None
+        if not texts:
+            return
+        if six.PY2:
+            return ' '.join(
+                [x.encode('utf-8', errors='ignore') for x in texts])
+        return ' '.join([str(x) for x in texts])
 
 
 def data_find_text_or_all(data, path, dyn_cls=False):