Use new sybil 3 name for codeblock parser if available

[bnavigator]

Hi, we are trying to enable packages for Python 3.10 in openSUSE Tumleweed. We need sybil 3 for that, but not being able to update testfixtures blocks it.

AFAICT the rename of the CodeBlockParser was the only relevant change. Edit: Did not check the doctests. My bad.

New version of #166: Also allow older version for Python 2 support.

[cjw296]

Hi Ben! Not sure what your timescales are, but there's a big new release of testfixtures brewing here.

That already has Sybil 3 in it, along with dropping Python 2 support, a documentation refresh and, hopefully, type annotations throughout.

I don't have fixed timescales on that, but probably early in the new year. Would that work for you?

[bnavigator]

Hi,

I was not aware that you prepare a new release in a separate fork. Whatever your release schedule is, the openSUSE package will follow. Until then, in order to keep the openSUSE package ecosystem in a sane state, we will use this patch to have both sybil 3 and testfixtures 6.18.3 coexisting.

I am closing this PR then, as cjw296@81981ef will be the more advanced change for the future release.

[cjw296]

testfixtures v7 is taking longer than I'd like, so let's see if we can land this.