Add `astunparse` as `spack_astunparse`. This library unparses Python ASTs and we're
adding it under our own name so that we can make modifications to it.
Ultimately this will be used to make `package_hash` consistent across Python versions.
`sbang` now lives at https://github.com/spack/sbang, and it has its own
test suite that's more extensive than what's in Spack. We'll leave sbang
tests to sbang from now on, and just vendor `bin/sbang` directly.
Remaining `sbang` tests have to do with patching files, not with
`sbang`'s functionality.
This update also fixes a bug with `sbang` and multiple command line
arguments that was introduced in #19529. See:
* https://github.com/spack/sbang/pull/1
* https://github.com/spack/sbang/pull/2
- [x] include latest `sbang` from https://github.com/spack/sbang
- [x] remove old `sbang` tests from Spack
- [x] update `COPYRIGHT` and `cmd/license.py`
