Feat: added setters for content property of block tokens

[uweschmitt]

I'm using mistletoe to transform markdown to markdown and added setters to the content properties of block token classes in block_tokens.py.

I did not add tests yet, since I did not find a suitable place in the existing setup.

[uweschmitt]

Btw: I realized that the code is not black conformant, although https://github.com/miyuchina/mistletoe/blob/master/CONTRIBUTING.md#style-guide recommends black.

[pbodnar]

@uweschmitt, thanks for the PR.

I did not add tests yet, since I did not find a suitable place in the existing setup.

What about extending the TestLeafBlockTokenContentProperty tests? A little bit of contemplation on how to (re)structure the tests will be probably needed...

[pbodnar]

Btw: I realized that the code is not black conformant, although https://github.com/miyuchina/mistletoe/blob/master/CONTRIBUTING.md#style-guide recommends black.

Yeah, the decision to use the black formatter came relatively recently. And I'm still not sure whether we want to re-format the whole code base by applying it globally.