Log Message:
Update to 15.2.1:

Twisted Core 15.2.1 (2015-05-23)
================================

Bugfixes
--------
 - twisted.logger now marks the `isError` key correctly on legacy
   events generated by writes to stderr. (#7903)

Improved Documentation
----------------------
 - twisted.logger's documentation is now correctly listed in the table
   of contents. (#7904)