Replace use of apptools sweetpickle

[aaronayres35]

Closes #45

This PR replaces the use of apptools sweetpickle with apptools.persistence and pickle from the standard library (as was done in enthought/apptools#199)

I believe the relevant code is covered by tests (e.g.

codetools/codetools/contexts/tests/data_context_test_case.py

Lines 28 to 43 in aa0d470

	def test_persistence():
	""" Can DataContexts round-trip through the persistence mechanism?
	"""
	d = DataContext(name='test_context')
	d['a'] = 1
	d['b'] = 2
	f = BytesIO()
	d.save(f)
	f.seek(0, 0)
	d2 = DataContext.load(f)
	assert d.name == d2.name
	assert set(d2.keys()) == set(['a', 'b'])
	assert d2['a'] == d['a']
	assert d2['b'] == d['b']

) , and running the test suite locally, tests all still passed.
As a sanity check it would be good to test with an application that depends on codetools DataContext

[rahulporuri]

LGTM. I tested this against an internal application and the application is now happy.

Looks like there's a test (test_persistence) which makes me slightly happy.

It'd also be useful if we could store a pickled DataContext in the codebase and run tests against it - to ensure that we're maintaing backwards compatibility in terms of storing/restoring the DataContext objects.

[aaronayres35]

It'd also be useful if we could store a pickled DataContext in the codebase and run tests against it - to ensure that we're maintaing backwards compatibility in terms of storing/restoring the DataContext objects.

Just to make sure I'm on the same page, so you mean we create a data context and save it to some file (this is a one time thing). Then we store that file in like tests/data or something, and in various tests, unpickle the object from that file and use it as the DataContext being tested?

Also should this be done in this PR, or a follow up PR and we push this through as is?

[rahulporuri]

Just to make sure I'm on the same page, so you mean we create a data context and save it to some file (this is a one time thing). Then we store that file in like tests/data or something, and in various tests, unpickle the object from that file and use it as the DataContext being tested?

Yup. That's exactly what i mean.

Also should this be done in this PR, or a follow up PR and we push this through as is?

We can push this through as is - but it'd be good to add that pickle/test sooner rather than later so we can move on and not have codetools in the back of our minds.