Flag problematic uses of blockquote (with lists, blocks etc)

[pradyunsg]

When writing reST, it is fairly easy to "silently" make a mistake in indentation that results in a blockquote being inserted in a list entry, especially when using nested lists. This would be similar to the check_bad_dedent case (whose example also involves creation of a blockquote).

[pradyunsg]

A few examples of this sort of mistake can be found in python/devguide#1178.

[ezio-melotti]

I'm working on a new checker that I used to fix a bunch of these errors in the following issues (and linked PRs):

• Fix unintentional blockquotes caused by wrong indentation python/devguide#1178
• Fix wrongly indented blocks in the documentation python/cpython#110631

I will submit a PR once it's ready.