:pythonx heredoc lines are parsed as Ex commands when skipping - #21422

What does this change do?

See commit message.

Commit message

Problem:  :pythonx heredoc lines are parsed as Ex commands when skipping.
Solution: Consume the heredoc lines like the other :python commands, and
          don't initialise 'pyxversion', when skipping. (Doug Kearns)

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Checklist

• The commit message follows the Problem/Solution form above
• Signed-off-by: trailer is present
• Tests were added
• No documentation update is needed

Anything reviewers should know

I'm aware of at least a dozen command skipping issues that I turned up when working on something a little while ago, and more have been introduced recently. I'll endeavour to tease them out over the next few days.

@chrisbra, I think the robustness of command skipping should have been better tested before :source-dryrun was merged. I'd consider backing it out but if that's going to cause too much of a kerfuffle it would be a good idea to add a warning in the help until it is more mature.