Fix EnsureListOf() and EnsureTupleOf() for string inputs

[nobodyinperson]

EnsureListOf() and EnsureTupleOf() took string inputs as iterables. This PR fixes this.

(I don't understand this scriv thing, scriv create etc. all error out with unfound files, so I'll leave it to you to add this CHANGELOG label.)

[codecov]

Codecov Report

Base: 88.72% // Head: 90.55% // Increases project coverage by +1.82% 🎉

Coverage data is based on head (e581732) compared to base (c92ae3a).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##            maint    #7267      +/-   ##
==========================================
+ Coverage   88.72%   90.55%   +1.82%     
==========================================
  Files         326      326              
  Lines       44423    44425       +2     
  Branches     5919        0    -5919     
==========================================
+ Hits        39415    40229     +814     
+ Misses       4993     4196     -797     
+ Partials       15        0      -15     

Impacted Files	Coverage Δ
datalad/support/constraints.py	89.95% <100.00%> (ø)
datalad/tests/test_constraints.py	100.00% <100.00%> (ø)
datalad/support/nda_.py	0.00% <0.00%> (-54.55%)	⬇️
datalad/customremotes/ria_remote.py	55.55% <0.00%> (-11.12%)	⬇️
datalad/cli/interface.py	97.22% <0.00%> (-2.78%)	⬇️
datalad/support/network.py	88.22% <0.00%> (-2.60%)	⬇️
datalad/support/s3.py	40.00% <0.00%> (-2.27%)	⬇️
datalad/utils.py	86.35% <0.00%> (-1.62%)	⬇️
datalad/local/remove.py	95.31% <0.00%> (-1.57%)	⬇️
datalad/support/json_py.py	97.56% <0.00%> (-1.22%)	⬇️
... and 15 more

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[yarikoptic]

Well spotted.
To reduce code duplication: I think in this both cases we could just use ensure_list or ensure_tuple in both cases from https://github.com/datalad/datalad/blob/HEAD/datalad/utils.py#L734.

[adswa]

This PR can be merged from my point of view

[yarikoptic]

Let's do it then. tuneups to reduce code duplication could be done later.

[yarikoptic-gitmate]

PR released in 0.18.2