Updates to cover style API changes

[springmeyer]

Moved here from https://github.com/mapnik/mapnik/issues/2324

Basically Mapnik 3.x overhauled how symbolizers work in core. This is for the better: they are now flexible key-value objects. But previously the python bindings modeled very closely the old symbolizer object structures. This means much of the symbolizer API is now not backwards compatible in 3.x. And overall needs a re-think so that we can expose styling properties in a way that does not radically change again and is easy to maintain and test.

expose styling properties in a way that does not radically change again and is easy to maintain and test.

Because ^^ is a major task. For the 3.0.0 release I think the most viable approach is:

• disable / comment out the symbolizer objects in the bindings for now
• recommend people upgrading to Mapnik 3.x use the XML styling interface instead

[alexandervlpl]

@artemp @springmeyer So 3 years later, which symbolizers are still broken? ¯\(ツ)/¯ I want to document this somehow (#184) or at least mention it on the old wiki (which is extremely misleading right now).