Fail Travis tests on deprecation warnings

[jwodder]

See datalad/datalad-registry#61 (comment)

[codecov]

Codecov Report

Merging #6074 (3ec47bb) into maint (62a8987) will decrease coverage by 32.72%.
The diff coverage is 43.00%.

❗ Current head 3ec47bb differs from pull request most recent head 22ba985. Consider uploading reports for the commit 22ba985 to get more accurate results

@@             Coverage Diff             @@
##            maint    #6074       +/-   ##
===========================================
- Coverage   90.24%   57.51%   -32.73%     
===========================================
  Files         312      312               
  Lines       42215    42473      +258     
===========================================
- Hits        38096    24429    -13667     
- Misses       4119    18044    +13925     

Impacted Files	Coverage Δ
datalad/cmdline/common_args.py	100.00% <ø> (ø)
datalad/cmdline/helpers.py	71.09% <0.00%> (ø)
datalad/core/distributed/tests/test_push.py	80.04% <ø> (-18.90%)	⬇️
...ad/distributed/tests/test_create_sibling_gitlab.py	0.00% <0.00%> (-100.00%)	⬇️
...talad/distributed/tests/test_create_sibling_ria.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/distributed/tests/test_ria.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/distributed/tests/test_ria_basics.py	0.00% <ø> (-97.57%)	⬇️
datalad/interface/__init__.py	100.00% <ø> (ø)
datalad/interface/base.py	90.00% <ø> (-0.29%)	⬇️
datalad/local/tests/test_subdataset.py	0.00% <0.00%> (-100.00%)	⬇️
... and 231 more

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[mih]

Thx!

I filed #6079, I don't see why this should be travis-only.