[TIP] Multiplatform testing

[Robert Collins]

There's also a helper in testtools for this:

    try_import(name, alternative=None)
        Attempt to import `name`.  If it fails, return `alternative`.

        When supporting multiple versions of Python or optional dependencies, it
        is useful to be able to try to import a module.

        :param name: The name of the object to import, e.g. 'os.path' or
            'os.path.join'.
        :param alternative: The value to return if no module can be imported.
            Defaults to None.

    try_imports(module_names, alternative=<object object>)
        Attempt to import modules.

        Tries to import the first module in `module_names`.  If it can be
        imported, we return it.  If not, we go on to the second module and try
        that.  The process continues until we run out of modules to
try.  If none
        of the modules can be imported, either raise an exception or return the
        provided `alternative` value.

        :param module_names: A sequence of module names to try to import.
        :param alternative: The value to return if no module can be imported.
            If unspecified, we raise an ImportError.
        :raises ImportError: If none of the modules can be imported and no
            alternative value was specified.