Removed unused support code for unittest2.
The unittest2 backport module is no longer
necessary since Python 3.3+, and the small amount of code in pytest to support it also doesn't seem
to be used: after removed, all tests still pass unchanged.
Although our policy is to introduce a deprecation period before removing any features or support
for third party libraries, because this code is apparently not used
at all (even if unittest2 is used by a test suite executed by pytest), it was decided to
remove it in this release.
If you experience a regression because of this, please file an issue.