Remove dependency on textfixtures

See https://github.com/coala/coala/issues/5335 for background.

0.7.0 included a dependency on a library intended for testing: testfixtures.

The introduction is 04a470cc , !52 (merged)

Removing this method must be done in 0.8.0 ; it can not be 0.7.x as that violates Semver.

While testfixtures might be fairly stable, and might have no mandatory dependencies now, should a problem arise they are not going to be impressed that it is being used in production low level code.

They currently have a lot of 'extra' dependencies, which are not installed by default. https://github.com/Simplistix/testfixtures/blob/master/setup.py#L36 They also have lots of code specific to testing with various frameworks which we dont use. No reason to have that all being forced on the user, especially without any pip wheel caching, slowing down installs.

One alternative is to find a smaller library which offers the same functionality, with zero dependencies and a high degree of maintainability. The only one I can find is https://github.com/quaddra/engage-utils/blob/master/engage_utils/log_utils.py , and I think it fails the maintainability criteria.

Failing that, building our own test functionality is reasonable, in a different library. Also capturing of logging isnt a testing-only util, so it could be added here, but so far the use cases are all testing.