* Replaced all from ...py23 import * with explicit name imports, or removed completely when possible.
* Replaced tounicode() with tostr()
* Changed all BytesIO ans StringIO imports to from io import ..., replaced all UnicodeIO with StringIO.
* Replaced all unichr() with chr()
* Misc minor tweaks and fixes
For reproducible builds, check the presence of SOURCE_DATE_EPOCH
environment variable and use it for the time stamp. This affects the
head.modified (and head.created in merge.py).
See https://reproducible-builds.org/specs/source-date-epoch/
