Unpin argcomplete version

[msto]

Hi,

Would it be possible to unpin the argcomplete dependency?

Thanks!

Motivation

We have an existing environment in which we'd like to use miniwdl, but there's a dependency conflict with the pinned version of argcomplete.

Approach

Loosening the dependency requirement to >=1.12 resolves the conflicts in our environment and doesn't appear to be breaking.

Checklist

• Add appropriate test(s) to the automatic suite
• Use make pretty to reformat the code with black
• Use make check to statically check the code using Pyre and Pylint
• Send PR from a dedicated branch without unrelated edits
• Ensure compatibility with this project's MIT license

[mlin]

@msto @geoffjentry Updated the argcomplete dependency in #662. I found the unit test needed some updates, and then this feature always needs manual/interactive testing, which I did. Thanks!