Ignore useless warning about Ska being already imported

[taldcroft]

Description

From within the source repo for a namespace package one gets a warning like below when importing:

In [1]: import Ska.Shell                                                                                
/Users/aldcroft/miniconda3/envs/ska3-shiny/lib/python3.7/site-packages/ska_helpers/version.py:13: UserWarning:
 Module Ska was already imported from None, but /Users/aldcroft/git/Ska.Shell is being added to sys.path

Testing

• [N/A] Passes unit tests on MacOS, linux, Windows (at least one required)
• Functional testing

With the patch in place the warning goes away.

[javierggt]

Normally one does not see this warning. Is this in the context of running testr?

I think the changes are ok, I just wonder whether this can be an issue later on.

[taldcroft]

The context is just importing the package in IPython.

BTW, this might go away if/when we transition to importlib.metadata, which is apparently also much faster.