Fix gnuplot --version encoding handling on Windows

[sandersaares]

In some configurations, gnuplot --version will emit output as UTF-16 bytes instead of UTF-8. This is not expected by parse_version() which always expects UTF-8.

This change enhances parse_version() with a simple fallback - if UTF-8 parsing fails, it will try again with UTF-16.

Fixes #570.

[sandersaares]

@bheisler @lemmih ping for attention. Would appreciate getting this merged to unblock the feature on Windows.

[samueltardieu]

@sandersaares If you could rebase your PR on the latest master, this would help to have a clean CI build.

[sandersaares]

Rebased