Release 3.0.0 rc3

Changes from 3.0.0-rc.2 to 3.0.0-rc.3

• Now you can get and set the whole values of VLMeta instances with the vlmeta[:] syntax.
  The get part is syntactic sugar for vlmeta.getall() actually.

• blosc2.copy() now honors cparams= parameter.

• Now, compiling the package with USE_SYSTEM_BLOSC2 envar set to 1 will use the
  system-wide Blosc2 library. This is useful for creating packages that do not want
  to bundle the Blosc2 library (e.g. conda).

• Several changes in the build process to enable conda-forge packaging.

• Now, blosc2.pack_tensor() can pack empty tensors/arrays. Fixes #290.