Move to DataLad 0.12

[mih]

• stop PY2 test runs
• strip direct mode failure decorators
• stop using obsolete Dataset.add()
• depend on DataLad 0.12

Companion to https://github.com/datalad/datalad/pull/4158

[codecov[bot]]

Codecov Report

Merging #70 into master will decrease coverage by 0.13%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   82.41%   82.27%   -0.14%     
==========================================
  Files          53       53              
  Lines        4253     4232      -21     
==========================================
- Hits         3505     3482      -23     
- Misses        748      750       +2

Impacted Files	Coverage Δ
...rawler/pipelines/tests/test_openfmri_collection.py	100% <ø> (ø)	:arrow_up:
datalad_crawler/pipelines/tests/test_openfmri.py	43.34% <ø> (-0.85%)	:arrow_down:
...awler/pipelines/tests/test_simple_with_archives.py	96.38% <ø> (-0.13%)	:arrow_down:
datalad_crawler/pipelines/tests/test_balsa.py	100% <ø> (ø)	:arrow_up:
datalad_crawler/nodes/annex.py	78.36% <100%> (ø)	:arrow_up:
datalad_crawler/nodes/tests/test_annex.py	98.8% <100%> (-0.81%)	:arrow_down:
datalad_crawler/pipelines/tests/test_crcns.py	70.83% <0%> (-4.17%)	:arrow_down:
datalad_crawler/nodes/tests/test_misc.py	100% <0%> (ø)	:arrow_up:
... and 1 more

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[mih]

Let's do this.