bug: Enabling pymdownx line numbers globally also renders them in signatures

[pawamoy]

Line numbers appear for signatures. They shouldn't (it doesn't make sense). Reported on the Matrix channel.

[pawamoy]

OK, easy fix. I also decided to do the opposite for pycon code examples in docstrings (the ones written without fences) and to start respecting highlight's configuration. Commit message:

The reasoning is that someone enabling line numbers globally supposedly also wants them for code blocks in docstrings. There's in my opinion no difference between a code block in a Markdown document and a code block in a docstring, they both serve documentation purposes and should be subject to the same rendering rules.

If that makes someone unhappy, we can always consider adding an option to hide line numbers for code blocks in docstrings.