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-packaging/package.py

@@ -31,6 +31,8 @@ class PyPackaging(PythonPackage):
     homepage = "https://github.com/pypa/packaging"
     url      = "https://pypi.io/packages/source/p/packaging/packaging-16.8.tar.gz"
 
+    import_modules = ['packaging']
+
     version('16.8', '53895cdca04ecff80b54128e475b5d3b')
 
     # Not needed for the installation, but used at runtime