Revert usage of setuptools enviroment markers (feature too new)

fix reference to pathlib from pypi

Fix #161 -- use extras_require to specify per-platform/per-Python dep…

…endencies

note on index that python2 support was dropped.

bump version to 0.29.0

bump version 0.28

bump to revision 0.27

pyflakes fix

bump release to 0.25

update docs, bump version 0.24