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-appdirs/package.py

@@ -30,8 +30,11 @@ class PyAppdirs(PythonPackage):
     dirs, e.g. a "user data dir"."""
 
     homepage = "https://github.com/ActiveState/appdirs"
-    url      = "https://pypi.io/packages/source/a/appdirs/appdirs-1.4.0.tar.gz"
+    url      = "https://pypi.io/packages/source/a/appdirs/appdirs-1.4.3.tar.gz"
 
+    import_modules = ['appdirs']
+
+    version('1.4.3', '44c679904082a2133f5566c8a0d3ab42')
     version('1.4.0', '1d17b4c9694ab84794e228f28dc3275b')
 
     patch('setuptools-import.patch', when='@:1.4.0')