util: prefer "bytesio" to "stringio" The io.BytesIO and io.StringIO types enforce the type of data being operated on. On Python 2, we use cStringIO.StringIO(), which is lax about mixing types. On Python 3, we actually use io.BytesIO. Ideally, we'd use io.BytesIO on Python 2. But I believe its performance is poor compared to cString.StringIO(). Anyway, we canonically define our pycompat type as "stringio." That name is misleading, especially on Python 3. This commit renames the canonical symbols to "bytesio." "stringio" is preserved as an alias for API compatibility. There are a lot of callers in the repo and I hesitate to take away the old name. I also don't feel like changing everything at this time. But at least new callers can use a "proper" name. Differential Revision: https://phab.mercurial-scm.org/D2868