Drop support to Plone 4.3 and 5.1

[wesleybl]

This PR cleans up code that is no longer needed in Plone >=5.2. Skip tests that fail in Plone 5.2.

There is still work to be done but I decided to stop here so the PR doesn't get too big.

[codecov-commenter]

Codecov Report

Merging #244 (6b004bd) into master (c123006) will decrease coverage by 14.32%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master     #244       +/-   ##
===========================================
- Coverage   96.35%   82.03%   -14.33%     
===========================================
  Files          33       31        -2     
  Lines        1399     1319       -80     
===========================================
- Hits         1348     1082      -266     
- Misses         51      237      +186     

Impacted Files	Coverage Δ
src/collective/nitf/setuphandlers.py	100.00% <ø> (ø)
src/collective/nitf/testing.py	65.67% <100.00%> (-29.74%)	:arrow_down:
src/collective/nitf/tests/test_collection.py	77.58% <100.00%> (-22.42%)	:arrow_down:
src/collective/nitf/tests/test_controlpanel.py	75.38% <100.00%> (-24.62%)	:arrow_down:
src/collective/nitf/tests/test_doctest.py	100.00% <100.00%> (ø)
src/collective/nitf/tests/test_indexing.py	92.40% <100.00%> (-7.60%)	:arrow_down:
src/collective/nitf/tests/test_robot.py	100.00% <100.00%> (+5.55%)	:arrow_up:
src/collective/nitf/tests/test_setup.py	88.09% <100.00%> (-7.15%)	:arrow_down:
src/collective/nitf/tests/test_viewlets.py	34.24% <100.00%> (-63.13%)	:arrow_down:
src/collective/nitf/tests/test_views.py	94.07% <100.00%> (-5.93%)	:arrow_down:
... and 13 more

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[wesleybl]

@hvelarde I made the change you asked for. I need to advance here. So I'm going to merge it.

Anything that is not ok, please let me know.