Remove additional backticks from inline code

[domdfcoding]

Unbalanced backticks appear in the rendered HTML, as do any additional backticks if there are more than two.

For example:

[gvanrossum]

@hugovk Should we update the linter to catch these?

[hugovk]

It might be a bit tricky to come up with a regex pattern to match. For example, how about three backticks in a row?

But PEP 612 uses backticks for valid section title underlines:

$ git grep "\`\`\`" pep-0612.rst
pep-0612.rst:````````````
pep-0612.rst:```````````````````
pep-0612.rst:````````````````````````````
pep-0612.rst:`````````
pep-0612.rst:```````````````````
pep-0612.rst:`````````

Then how about three backticks with a non-backtick on one side? But PEPs 501 and 502 have these:

$ git grep "\`\`\`" pep-050*.txt
pep-0501.txt:only with the backtick based ``\`cat $filename\``` syntax replaced by
pep-0502.txt:such as the "Backtick" (or grave accent `````) are also

[gvanrossum]

Okay, fair. But it makes me think we should perhaps invest in a proper linter.