py-netCDF4: updated to 1.5.6

version 1.5.6 (tag v1.5.6rel)
=============================
* move CI/CD tests from travis/appveyor to Github Actions.
* move netCDF4 dir under src so module can be imported in source directory.
* change numpy.bool to numpy.bool_ and numpy.float to numpy.float_ (float and
  bool are deprecated in numpy 1.20)
* clean up docstrings so that they work with latest pdoc.
* update cython numpy API to remove deprecation warnings.
* Add "fromcdl" and "tocdl" Dataset methods for import/export of CDL
  via ncdump/ncgen called externally via the subprocess module.
* remove python 2.7 support.
* broadcast data (if possible)to conform to variable shape when writing to a slice

version 1.5.5.1 (tag v1.5.5.1rel)
=================================
* rebuild binary wheels for linux and OSX to link netcdf-c 4.7.4 and hdf5 1.12.0.

version 1.5.5 (tag v1.5.5rel)
=============================
* have setup.py always try use nc-config first to find paths to netcdf and
  hdf5 libraries and headers. Don't use pkg-config to find HDF5 if HDF5 env
  vars are set (or read from setup.cfg).
* Change MIT license text to standard OSI wording.

version 1.5.4 (tag v1.5.4rel)
=============================
* fix printing of variable objects for variables that end with the letter 'u'
* make sure root group has 'name' attribute.
* add the ability to pack vlen floats to integers using
  scale_factor/add_offset
* use len instead of deprecated numpy.alen
* check size on valid_range instead of using len.
* add `set_chunk_cache/get_chunk_cache` module functions to reset the
  default chunk cache sizes before opening a Dataset.
* replace use of numpy's deprecated tostring() method with tobytes()
* bump minimal numpy version to 1.9 (first version to have tobytes()).


(adam)