When adding support for ProofreadPage to pywikibot (https://gerrit.wikimedia.org/r/#/c/195105 by @Mpaa), we've found that this extension doesnt expose a version number to the API.

e.g. https://en.wikisource.org/wiki/Special:Version only says "– (7709950) 2015-04-20T18:41:29"

As some features of this extension are not included in the MW 1.19 branch, and will not be included, we will need to use either version or feature detection in the future if anyone is using pywikibot , ProofreadPage and MW 1.19. Extension version detection is much easier than feature detection.