Question: is non-utf8 supported on Python 3?

[mauritsvanrees]

While creating PR #110, I ran into code and tests that tried to support other encodings, for example iso-8859-1, at least when importing an XML file. But some parts of the code ignore this on Python 3.

So I wonder: does GenericSetup on Python 3 only support utf-8?
If so, then some code could be simplified, for example the code in that PR.
But simplifying may need to wait until we drop support for Python 2, and/or we require Zope 5.3, where lines properties are text instead of bytes.

I wonder the same about Zope 5: does that only support bytes encoded as utf-8, at least when stored in the Data.fs?

Note: since Plone 4.3, core Plone assumes all bytes that are read from Data.fs are utf-8 encoded (or simple ascii), where previously you could set a default_charset property (preferably only set it once, never change it). See this commit from 2012.