Deprecate traits.testing.nose_tools

[mdickinson]

We don't test the machinery in traits.testing.nose_tools, but there are other parts of ETS that still use it. Those users are likely no longer using nose to run their test suites.

EDIT: I'm only seeing uses of nose_tools in etsproxy, which we don't care about at this point. No other uses of nose_tools in ETS. Still, there might be external uses that we don't know about, so we can't simply delete without warning.

Related: #852.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@63b138f). Click here to learn what that means.
The diff coverage is 71.42%.

@@            Coverage Diff            @@
##             master     #880   +/-   ##
=========================================
  Coverage          ?   72.58%           
=========================================
  Files             ?       51           
  Lines             ?     6375           
  Branches          ?     1278           
=========================================
  Hits              ?     4627           
  Misses            ?     1355           
  Partials          ?      393

Impacted Files	Coverage Δ
traits/testing/nose_tools.py	60.71% <71.42%> (ø)

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