Fix parsing of "PHP-style" versions in version_tuple_from_generator

In the examples in the MediaWiki documentation, versions are compared
using the `version_compare` function from the PHP standard library:

https://www.mediawiki.org/wiki/Manual:$wgVersion#Example_code

Before comparison, this version canonicalizes version strings according
to the following rules:

- Replace _, - and + with a dot .
- Insert dots . before and after any non number

This commit updates `Site.version_tuple_from_generator` to parse
versions according to those rules.