Minor fixes

[bpoldrack]

• Closes #158
• raise dependency on datalad-neuroimaging to deal with pydicom 2.0.0
• raise dependency on datalad to >=0.13.4
• use cached datasets
• github action to test against datalad master
• use locally configured source for toolbox in tests too

[codecov-commenter]

Codecov Report

Merging #160 into master will decrease coverage by 1.94%. The diff coverage is 73.04%.

@@            Coverage Diff             @@
##           master     #160      +/-   ##
==========================================
- Coverage   86.17%   84.23%   -1.95%     
==========================================
  Files          17       19       +2     
  Lines         955     1078     +123     
==========================================
+ Hits          823      908      +85     
- Misses        132      170      +38     

Impacted Files	Coverage Δ
datalad_hirni/tests/utils.py	51.31% <51.31%> (ø)
datalad_hirni/tests/test_dicom2spec.py	99.25% <95.65%> (-0.75%)	:arrow_down:
datalad_hirni/commands/import_dicoms.py	92.85% <100.00%> (+0.07%)	:arrow_up:
datalad_hirni/tests/__init__.py	100.00% <100.00%> (ø)
datalad_hirni/tests/test_demos.py	93.67% <100.00%> (+0.81%)	:arrow_up:
datalad_hirni/tests/test_dicomconv.py	98.50% <100.00%> (+0.32%)	:arrow_up:
datalad_hirni/tests/test_import.py	100.00% <100.00%> (ø)

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