Document that Django 1.11.x raises a deprecation warning with Python 3.7

[אורי]

1. ​https://docs.djangoproject.com/en/dev/releases/1.11/ should contain something like "running Django 1.11.* with Python 3.7 with deprecation warnings enabled will raise a DeprecationWarning." or similar (#30078).

2. Change from 1.11.17 to 1.11.18.

3. Change "will end in April 2018." to "ended in April 2018."

Same for all versions of this doc such as ​https://docs.djangoproject.com/en/1.11/releases/1.11/.

[Tim Graham]

1. I don't see value in documenting this.
2. The documentation is correct that Python 3.7 support was added in Django 1.11.17.
3. There isn't much value in this change and we avoid trivial updates because it requires translators to update translated docs (and versions other the the latest stable version of the docs, Django 2.1 as of now, aren't retranslated).

[Dylan Young]

NOTE that this isn't actually removed until version 3.10, so it shouldn't bite anyone in any case. (See python 3.9 docs for the collections module)