Deprecation warning with Numpy v1.19.0

[scotthavens]

Numpy was upgraded to version 1.19.0 and it's throwing a deprecation warning for the spatial ref

spatialnc/proj.py:121: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
  map_meta = parse_wkt(ds['projection'].getncattr('spatial_ref'))
/home/scotthavens/code/smrf/smrf/output/output_netcdf.py:165: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.

Solution will be to use numpy<1.19.0 in the requirements.