Use pre-3.10 union syntax in WDL runner

[adamnovak]

This fixes #5041 by using syntax that older Pythons can actually understand when talking about union types in executable code. It turns out there wasn't a race; the #5030 PR that introduced the problem changed more than just the dependency version, and we didn't test it on older Python versions as part of the PR CI pipeline.

Changelog Entry

To be copied to the draft changelog by merger:

• toil-wdl-runner works again on Python 3.8 and 3.9 after MiniWDL version upgrade

Reviewer Checklist

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