announce: make _chunks() helper safe against more input types

Especially `dict_keys` objects, which is how py3 handles `dict.keys()`
(vs. being a subscriptable iterable in py2).

Added a test, too, because let's not have this plugin bite us again.

NOTE:
Replaced the new test file's `__future__` imports with those appropriate
for py2, and added the magic "coding" comment expected by our 7.x style
checker rules. Backported from d915136.

sopel/modules/announce.py

@@ -18,10 +18,18 @@ def _chunks(items, size):
     :param items: the collection of items to chunk
     :type items: :term:`iterable`
     :param int size: the size of each chunk
+    :return: a :term:`generator` of chunks
+    :rtype: :term:`generator` of :class:`tuple`
     """
+    # Need to convert non-subscriptable types like `dict_keys` objects
+    try:
+        items[0]
+    except TypeError:
+        items = tuple(items)
+
     # from https://stackoverflow.com/a/312464/5991 with modified names for readability
     for delim in range(0, len(items), size):
-        yield items[delim:delim + size]
+        yield tuple(items[delim:delim + size])
 
 
 @plugin.command('announce')

test/modules/test_modules_announce.py

@@ -0,0 +1,33 @@
+# coding=utf-8
+"""Tests for Sopel's ``announce`` plugin"""
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from sopel.modules import announce
+
+
+def test_chunks():
+    """Test the `_chunks` helper for functionality and compatibility."""
+    # list input
+    items = ['list', 'of', 'items', 'to', 'chunk']
+    r = list(announce._chunks(items, 2))
+
+    assert len(r) == 3
+    assert r[0] == tuple(items[:2])
+    assert r[1] == tuple(items[2:4])
+    assert r[2] == tuple(items[4:])
+
+    # tuple input
+    items = ('tuple', 'of', 'items')
+    r = list(announce._chunks(items, 3))
+
+    assert len(r) == 1
+    assert r[0] == items
+
+    # dict keys input
+    keys = {'one': True, 'two': True, 'three': True}.keys()
+    items = list(keys)
+    r = list(announce._chunks(keys, 1))
+
+    assert len(r) == 3
+    for idx in range(len(items)):
+        assert r[idx] == (items[idx],)