Rework Spack's Mercurial support (#3834)

* Add tests to mercurial package

* Add support for --insecure with mercurial fetching

* Install man pages and tab-completion scripts

* Add tests and latest version for all deps

* Flake8 fix

* Use certifi module to find CA certificate

* Flake8 fix

* Unset PYTHONPATH when running hg

* svn_fetch should use to svn-test, not hg-test

* Drop Python 3 support in Mercurial

Python 3 support is a work in progress and isn't currently
recommended:

https://www.mercurial-scm.org/wiki/SupportedPythonVersions

* Test both secure and insecure hg fetching

* Test both secure and insecure git and svn fetching

var/spack/repos/builtin/packages/py-six/package.py

@@ -29,7 +29,9 @@ class PySix(PythonPackage):
     """Python 2 and 3 compatibility utilities."""
 
     homepage = "https://pypi.python.org/pypi/six"
-    url      = "https://pypi.io/packages/source/s/six/six-1.9.0.tar.gz"
+    url      = "https://pypi.io/packages/source/s/six/six-1.10.0.tar.gz"
+
+    import_modules = ['six']
 
     version('1.10.0', '34eed507548117b2ab523ab14b2f8b55')
     version('1.9.0',  '476881ef4012262dfc8adc645ee786c4')