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

[dgw]

Description

Fixes #2165.

Python 3 dict.keys() returns a dict_keys object, which is not subscriptable unlike the return value in py2.

Also made the generator return a consistent type (always tuple), and updated the documentation (not that it's actually used anywhere by Sphinx). Added a test, too, because let's (try to) not have this plugin bite us again.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
  • I ran both the new test and make quality on both py3(.8) and py2(.7), and the patch should be clean against both. Since this can't be a straight cherry-pick back to 7.1.x due to the new test file needing different headers, I have pre-prepped an announce-py2-backport branch for myself with the appropriate code style.
• I have tested the functionality of the things this change touches

[Exirel]

What about any of these 2 versions to get some chunk?

Option 1: use itertools and iter, and keep everything immutable:

import itertools

def chunk(items, size):
    iterator = iter(items)
    data = tuple(itertools.islice(iterator, size))
    while data:
        yield data
        data = tuple(itertools.islice(iterator, size))

And this version uses a mutable list, but it might be easier to read when you don't know what itertools and iter can do for you:

def chunk(items, size):
    acc = []
    for i, item in enumerate(items, start=1):
        acc.append(item)
        if i % size == 0:
            yield tuple(acc)
            acc = []
    if acc:
        yield tuple(acc)

[dgw]

I like the itertools one, but am too tired to figure out why when I forgot to wrap the call in tuple() it looped forever.

>>> while chunk := itertools.islice(iterator, 3):
...     print(chunk)
...
<itertools.islice object at 0x7f3d2b211400>
<itertools.islice object at 0x7f3d2b0e19a0>
<itertools.islice object at 0x7f3d2b211400>
[many more repetitions]
<itertools.islice object at 0x7f3d2b0e19a0>
<itertools.islice object at 0x7f3d2b211400>
<itertools.islice object at 0x7f3d2b0e19a0>
^CTraceback (most recent call last):
  File "<stdin>", line 2, in <module>
KeyboardInterrupt
>>> while chunk := tuple(itertools.islice(iterator, 3)):
...     print(chunk)
...
(1, 2, 3)
(4, 5, 6)
(7, 8)

(Note that now I'm drunk on dropping old Python versions and wish it was reasonable to support only 3.8+ already because the := assignment operator is 👌.)

[Exirel]

but am too tired to figure out why

Because islice returns a generator, which always evaluates to True. When you convert into a tuple, you actually get an empty or a non-empty tuple, which evaluates as False or True, allowing the while loop to stop its iteration.