Turn UserWarnings into errors

[Carreau]

And turn then back into only warnings into relevant tests.
This ensure that we are not using deprecated functionalities, except
when testing for it.

[pep8speaks]

Hello @Carreau! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-11-23 13:37:42 UTC

[codecov]

Codecov Report

Merging #639 (ab9b4c3) into master (b9bef8e) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #639   +/-   ##
=======================================
  Coverage   99.94%   99.94%           
=======================================
  Files          32       32           
  Lines       11203    11204    +1     
=======================================
+ Hits        11197    11198    +1     
  Misses          6        6           

Impacted Files	Coverage Δ
zarr/tests/test_creation.py	100.00% <100.00%> (ø)

[joshmoore]

  D:\a\zarr-python\zarr-python\zarr\n5.py:417: RuntimeWarning: Not all N5 implementations support blosc compression (yet). You might not be able to open the dataset with another N5 library.
    warnings.warn(

c:\miniconda\envs\zarr-env\lib\site-packages\ipytree\tree.py:43
  c:\miniconda\envs\zarr-env\lib\site-packages\ipytree\tree.py:43: DeprecationWarning: metadata {'trait': <traitlets.traitlets.Instance object at 0x000001ED3D3C4D60>} was set from the constructor. With traitlets 4.1, metadata should be set using the .tag() method, e.g., Int().tag(key1='value1', key2='value2')
    nodes = Tuple(trait=Instance(Widget)).tag(

[joshmoore]

Looks like this was already rolled in elsewhere.