init: remove dependency on spack.version

- no longer require `spack_version` to be a Version (it isn't used that
  way anyway)

- use a simple tuple `spack_version_info` with major, minor, patch
  versions

- generate `spack_version` from the tuple

lib/spack/docs/conf.py

@@ -192,9 +192,9 @@ def setup(sphinx):
 # built documents.
 #
 # The short X.Y version.
-version = str(spack.spack_version.up_to(2))
+version = '.'.join(str(s) for s in spack.spack_version_info[:2])
 # The full version, including alpha/beta/rc tags.
-release = str(spack.spack_version.up_to(2))
+release = spack.spack_version
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.