Always include release dates in changelog

[impredicative]

The changelog currently doesn't show the respective release dates. It is important to be able to see the dates right there.

As good examples, see:

• https://github.com/tkem/cachetools/blob/master/CHANGELOG.rst
• https://github.com/kurtmckee/feedparser/blob/develop/CHANGELOG.rst

The dates can be backfilled too.

[TahirJalilov]

done changelog