re-introduced auto clearing of mail.outbox (pytest-dev#434)

rst fix

mailoutbox fixups (pytest-dev#427)

Make sure `mail.outbox == foo`, `mail.outbox != bar` and `iter(mail.outbox)` raises exceptions.

This commit also improves the readability of the provided assertion message by hiding the internal traceback and showing the documentation link.

Updated changelog for 3.0

Changelog for 2.9.1

Document Django version support in changelog

2.8.0 version bump

bump the version for release of 2.6.2

update changelog+corrected version number for new release

Prepare for 2.1.0 release