Insufficient indentation for nested lists

Hello! I've been using the markdownify library in my app for a long time and it's been working great, but recently I updated and I noticed that the the interpretation for nested lists had changed and it messed up my workflow.

My workflow looks like this:

• import content as HTML
• converting the HTML to markdown for editing (using markdownify)
• editors can edit the content
• once editing is finished, convert markdown back to HTML for publishing (using Markdown)

I noticed that since this commit, markdownify changed from using tabs for list indentation (/t) to using spaces, which looks like 2 spaces for unordered lists and 3 spaces for ordered lists.

Since Markdown expects nested lists to be indented by 4 spaces, the nested lists output by markdownify aren't currently interpreted by Markdown.

Here's an example of what I mean:

Nested unordered list

original HTML	markdownify output	final HTML (using Markdown)
original list has 4 levels of indentation	only 2 spaces used for nested list indentation	final list has 2 levels of indentation

Nested ordered list

original HTML	markdownify output	final HTML (using Markdown)
original list has 4 levels of indentation	only 3 spaces used for nested list indentation	final list has 3 levels of indentation

I recently created a PR in my app to resolve this by overriding convert_li. If you are interested, I could do a PR on this repo as well.

On the other hand, if there's some configuration option that I am missing that resolves this or Markdown is too-strictly interpreting the spec and you don't believe this is a real issue, that's fine too.

Start at the convert_li entry point and inspect the indentation change referenced in commit c13bdd5. Reproduce nested unordered and ordered lists through markdownify and Markdown, then verify that nested levels round-trip with the expected four-space indentation.