Buildbot expects all strings used internally to be valid Unicode strings - not bytestrings.
Note that Buildbot rarely feeds strings back into external tools in such a way that those strings must match. For example, Buildbot does not attempt to access the filenames specified in a Change. So it is more important to store strings in a manner that will be most useful to a human reader (e.g., in logfiles, web status, etc.) than to store them in a lossless format.
On input, strings should be decoded, if their encoding is known. Where necessary, the assumed input encoding should be configurable. In some cases, such as filenames, this encoding is not known or not well-defined (e.g., a utf-8 encoded filename in a latin-1 directory). In these cases, the input mechanisms should make a best effort at decoding, and use e.g., the errors='replace' option to fail gracefully on un-decodable characters.
At most points where Buildbot outputs a string, the target encoding is known. For example, the web status can encode to utf-8. In cases where it is not known, it should be configurable, with a safe fallback (e.g., ascii with errors='replace'. For HTML/XML outputs, consider using errors='xmlcharrefreplace' instead.