Be compatible with eliot-tree 17.0.0 & Twisted >= 17.1.0

[exarkun]

Something unicode-y happened and eliot trees now get rendered uglyfuly in the testtools details output.

[exarkun]

As I understand it:

• eliottree started including some non-ascii in the tree whereas previously the tree was entirely ascii
• the eliottree becomes a text/utf Content detail of a Fixture
• testtools gathers the Content and creates a _StringException with its as_text
• testtools tries to add that _StringException as an error to the test result
• the test result is a twisted.trial.reporter.TreeReporter
• TreeReporter inherits addError from twisted.trial.reporter.Reporter
• Reporter.addError creates a Failure from the _StringException
• The Failures are all reported using Reporter._formatFailureTraceback
• _formatFailureTraceback calls Failure.getTraceback
• ... which calls Failure.printTraceback
• ... which eventually calls reflect.safe_str(self.value) to format the exception into the traceback
• safe_str does unicode(value).encode('ascii', 'backslashreplace') which smashes eliottree's nice unicode drawing characters into ugly useless unicode escapes.

[exarkun]

It turns out this is also related to Twisted 17.1 (or perhaps originally 16.6) in which the backslashreplace encoding was added to safe_str.