Release 1.21.0

Changes from 1.20.1 to 1.21.0

• Updated zstd codec to 1.4.8.

• Updated lz4 codec to 1.9.3.

• New instructions on how to use the libraries in python-blosc wheels
  so as to compile C-Blosc applications. See:
  https://github.com/Blosc/c-blosc/blob/master/COMPILING_WITH_WHEELS.rst