Replace askyesno() with click.confirm()

[jwodder]

It turns out that askyesno() is already provided by a library we're already using. The only advantage of the former is that it supports repeated prompting when no default is given and the user enters a empty line, but we weren't using that feature anyway.

[codecov]

Codecov Report

Merging #296 (07d8885) into master (c7aac72) will decrease coverage by 0.17%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #296      +/-   ##
==========================================
- Coverage   82.13%   81.95%   -0.18%     
==========================================
  Files          55       53       -2     
  Lines        4640     4594      -46     
==========================================
- Hits         3811     3765      -46     
  Misses        829      829              

Flag	Coverage Δ
unittests	81.95% <100.00%> (-0.18%)	⬇️

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Impacted Files	Coverage Δ
dandi/girder.py	87.88% <100.00%> (ø)
dandi/tests/test_girder.py	100.00% <100.00%> (ø)

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[yarikoptic]

and there would be no side-effect of some kind if we use dandi as python package, and thus not going through click's cmdline parsing/handling? just want to make sure

[jwodder]

@yarikoptic As far as I'm aware, the only "tie-in" the function has to the rest of Click is that uses a Click-specific exception if the user hits Cntrl-C.

[yarikoptic]

Hm, not entirely 100% optimal (outside code, e.g. soon to be introduced https://github.com/datalad/datalad/blob/master/datalad/support/parallel.py) might need to catch that exception too then in some very rare cases when underlying dialog happens to be embedded. But I think it is tolerable and we could later shim it if need arises. Let's proceed