Remove support for Python 2.6 (#27256)

Modifications:
- [x] Removed `centos:6` unit test, adjusted vermin checks
- [x] Removed backport of `collections.OrderedDict`
- [x] Removed backport of `functools.total_ordering`
- [x] Removed Python 2.6 specific skip markers in unit tests
- [x] Fixed a few minor Python 2.6 related TODOs in code

Updating the vendored dependencies will be done in separate PRs

lib/spack/external/__init__.py

@@ -17,7 +17,7 @@
 --------
 
 * Homepage: https://pypi.python.org/pypi/argparse
-* Usage: We include our own version to be Python 2.6 compatible.
+* Usage: We include our own version to be Python 3.X compatible.
 * Version: 1.4.0
 * Note: This package has been slightly modified to improve
   error message formatting. See the following commit if the
@@ -39,15 +39,6 @@
 * Usage: Provides a more stable linux distribution detection.
 * Version: 1.0.4 (last version supporting Python 2.6)
 
-functools
----------
-
-* Homepage: https://github.com/python/cpython/blob/2.7/Lib/functools.py
-* Usage: Used for implementation of total_ordering.
-* Version: Unversioned
-* Note: This is the functools.total_ordering implementation
-  from Python 2.7 backported so we can run on Python 2.6.
-
 jinja2
 ------
 
@@ -73,14 +64,6 @@
 * Usage: Implements a XML/HTML/XHTML Markup safe string for Python.
 * Version: 1.0
 
-orderddict
-----------
-
-* Homepage: https://pypi.org/project/ordereddict/
-* Usage: A drop-in substitute for Py2.7's new collections.OrderedDict
-  that works in Python 2.4-2.6.
-* Version: 1.1
-
 py
 --