Feature: support uvloop as a faster alternative to Python's default IO loop

[jhamman]

Adds support for uvloop as a faster alternative to Python's default IO loop.

closes #3451

TODO:

• [x] Add unit tests and/or doctests in docstrings
• [x] Add docstrings and API docs for any new/modified user-facing classes and functions
• [x] New/modified features documented in docs/user-guide/*.rst
• [x] Changes documented as a new file in changes/
• [x] GitHub Actions have all passed
• [ ] Test coverage is 100% (Codecov passes)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 61.27%. Comparing base (62551c7) to head (2230848). :warning: Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/zarr/core/sync.py	82.35%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3452      +/-   ##
==========================================
+ Coverage   61.24%   61.27%   +0.02%     
==========================================
  Files          83       83              
  Lines        9907     9922      +15     
==========================================
+ Hits         6068     6080      +12     
- Misses       3839     3842       +3     

Files with missing lines	Coverage Δ
src/zarr/core/config.py	29.16% <ø> (ø)
src/zarr/core/sync.py	63.93% <82.35%> (+2.25%)	:arrow_up:

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[d-v-b]

is there a good way to demo the performance difference between uvloop and asyncio? If so, would it make sense to include that demo in the examples directory?

[DimitriPapadopoulos]

Note that asyncio seems to have been improved in recent versions of Python, almost filling the gap with uvloop (see https://github.com/MagicStack/uvloop/issues/566 and https://github.com/MagicStack/uvloop/pull/644).