make EasyConfigParser.get_config_dict return a copy rather than a reference

[boegel]

fixes #3691

[Flamefire]

I would have done it at a higher level (i.e. in the parser which has-a format object to avoid duplicating this, but this works too. Thanks!

For future reference as written in Slack:
The faster solution would be to copy the dict only on (potential) modification, e.g. when it is assigned into the EC (set_keys) because now we copy even if we only read from it (e.g. in the EC test suite). However it was decided to go for the safer solution and avoid the potential for accidental modification at the cost of a (comparatively cheap) copy of a dict

[migueldiascosta]

lgtm

[migueldiascosta]

Going in, thanks @boegel!