Deprecate blosc helper functions

[dstansby]

As a pre-requisite for depending on pre-built blosc wheels (see #571, #262, #464), this deprecates several functions. The idea would be to have one deprecation cycle (say numdocecs 0.14), then remove the public functions in the next release (say numcodecs 0.15).

This uses the deprecated library to handle the deprecations, which does add a nother dependency, but it is quite lightweight.

[normanrz]

I guess decorators don't work in .pyx files?

[dstansby]

They probably do, but the idea here was to leave a non-deprecated version of the functions (starting with _) that we can call internally, and a deprecated version which has the same name as previous.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.92%. Comparing base (c5cf0c5) to head (c2d10a5).
Report is 25 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #619   +/-   ##
=======================================
  Coverage   99.92%   99.92%           
=======================================
  Files          62       62           
  Lines        2754     2754           
=======================================
  Hits         2752     2752           
  Misses          2        2           

Files with missing lines	Coverage Δ
numcodecs/__init__.py	100.00% <100.00%> (ø)
numcodecs/tests/test_blosc.py	100.00% <100.00%> (ø)

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[dstansby]

Last call for comments - if no-one objects, I'll merge this in a couple of weeks.