Add a DeprecationWarning for configure_traits() pickles

[jwiggins]

Somewhat related to #787, we should deprecate the use of pickles for saving state between invocations of configure_traits.

[codecov-io]

Codecov Report

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The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #792   +/-   ##
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  Coverage          ?   72.07%           
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  Files             ?       51           
  Lines             ?     6346           
  Branches          ?     1279           
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  Hits              ?     4574           
  Misses            ?     1377           
  Partials          ?      395

Impacted Files	Coverage Δ
traits/has_traits.py	70.38% <100%> (ø)

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[mdickinson]

Thanks. Do you have bandwidth to add a test for the deprecation? (Just an assertWarns should be enough.)

[jwiggins]

Do you have bandwidth to add a test for the deprecation?

Possibly not today. How close is the next release?

[mdickinson]

How close is the next release?

Not yet determined, but we may target the end of next week. Not today, anyway.

[mdickinson]

LGTM, but needs a test.

[mdickinson]

I added the appropriate self.assertWarns to existing tests.

[jwiggins]

Thanks for finishing this!