bug fix for help from typing package, add test for optional and union

[Jemeljanov]

Closes #508

Fixed bug where objects from typing instance were not displayed correctly mentioned in #508 and also Union types.

Added tests for typing.Union and typing.Optional

Other notes

Removed comment as the edge case mentioned is handled by new code.

Used arg_type = repr(arg_type).replace('typing.', '') to output a shorter message for user

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[dbieber]

Thanks for putting this together! At a quick glance, this LGTM. One comment inline above.

[Jemeljanov]

@dbieber it was a little bit different behaviour now it will be Optional[Union[int, str]], the Optional is because the default is None.

Now it is more in compliance with the typing in python code and a more robust version than with regular expression.

Additional check is to prevent the Optional repetition (one added from python-fire the other from typing package).

[dbieber]

I only just enabled CI for pull requests so this didn't surface earlier -- it looks like there are some test failures.