Release 2.20.0

Changes from 2.19.1 to 2.20.0

• Extended b2nd_expand_dims() to allow for list of axes like NumPy.
  See PR #676. This represents a breaking change for the API, but as
  it was introduced in 2.18.0, it is unlikely to be used in practice.
  Thanks to Luke Shaw (@lshaw8317).

• Fix detection of AVX512 in MSVC. See PR #678.
  Thanks to Theodore Tsirpanis (@teo-tsirpanis).

• Add unified Blosc2::blosc2 CMake target. See PR #677.
  Thanks to Theodore Tsirpanis (@teo-tsirpanis).

• Rename windows pthread functions and blosc_pthread_{xyz}. Fixes #673
  (Duplicate symbols for pthread functions on Windows).
  Thanks to Barak Ugav (@barakugav).