add release announcement

Regendocs

Fix reference to _CallOutcome in docs

finalize changelog for 2.8.7

2.8.6 release: version, CHANGELOG

Remove note about expected failing envs in tox, as tox now supports
skipping certain environments based on the platform.

Remove plugins_index generation from HOWTORELEASE

correct copy&paste error in the release announcement version number

refer more explicit to osx el cap in the changelogs

Merge branch '2.8.2-release'

Merge pull request pytest-dev#1084 from hpk42/branch-281

2.8.1 release