yaml: use ruamel.yaml instead of pyyaml

- ruamel.yaml allows round-tripping comments from/to files
- ruamel.yaml is single-source, python2/python3 compatible

lib/spack/external/__init__.py

@@ -118,12 +118,17 @@
   vendored copy ever needs to be updated again:
   https://github.com/spack/spack/pull/6801/commits/ff513c39f2c67ff615de5cbc581dd69a8ec96526
 
-pyyaml
+ruamel.yaml
 ------
 
-* Homepage: https://pypi.python.org/pypi/PyYAML
-* Usage: Used for config files.
-* Version: 3.12
+* Homepage: https://yaml.readthedocs.io/
+* Usage: Used for config files. Ruamel is based on PyYAML but is more
+  actively maintained and has more features, including round-tripping
+  comments read from config files.
+* Version: 0.11.15 (last version supporting Python 2.6)
+* Note: This package has been slightly modified to improve Python 2.6
+  compatibility -- some ``{}`` format strings were replaced, and the
+  import for ``OrderedDict`` was tweaked.
 
 six
 ---