* 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
* Revert most of "Use Vector in some places where it improves the clarity of the code (#2206)"
This reverts most of commit 0d3ce2cafc6f604fc46d4d3bfbab34a5bc59e27b.
* Use Vector in some places where it improves the clarity of the code
* add __all__ to vector.py
* turned some list comprehensions into generator expressions: there's no need for an intermediate list in these cases
* Add empty __slots__ to Vector, so we don't waste space on a __dict__.
* add some tests for segmentPointAtT
