Log Message:
2.8.1:
- p599: fix problems with implementation of 515.
         Substitutions from other sections were not made anymore if they were \ 
not in `envlist`.

2.8.0:
- 276: Remove easy_install from docs (TL;DR: use pip).
- 301: Expand nested substitutions in tox.ini
- 315: add --help and --version to helptox-quickstart
- 326: Fix OSError 'Not a directory' when creating env on Jython 2.7.0.
- 429: Forward MSYSTEM by default on Windows
- 449: add multi platform example to the docs.
- 474: Start using setuptools_scm for tag based versioning.
- 484: Renamed `py.test` to `pytest` throughout the project
- 504: With `-a`: do not show additional environments header if there are none
- 515: Don't require environment variables in test environments where they
       are not used.
- 517: Forward NUMBER_OF_PROCESSORS by default on Windows to fix
       `multiprocessor.cpu_count()`.
- 518: Forward `USERPROFILE` by default on Windows.
- p528: Fix some of the warnings displayed by pytest 3.1.0.
- p547: Add regression test for 137
- p553: Add an XFAIL test to reproduce upstream bug 203
- p556: Report more meaningful errors on why virtualenv creation failed
- 575: Add announcement doc to end all announcement docs
       (using only CHANGELOG and Github issues since 2.5 already)
- p580: Do not ignore Sphinx warnings anymore
- 585: Expand documentation to explain pass through of flags from deps to pip
       (e.g. -rrequirements.txt, -cconstraints.txt)
- 588: Run pytest wit xfail_strict and adapt affected tests.