- remove the old LGPL license headers from all files in Spack
- add SPDX headers to all files
- core and most packages are (Apache-2.0 OR MIT)
- a very small number of remaining packages are LGPL-2.1-only
* Add installer for petsc4py and slepc4py
* @adamjstewart requested changes on this pull request; make these type=('build', 'run')
* @citibeth suggestion Remove unneeded indirect dependencies