Pin importlib-metadata version in nwm_client_new

[aaraney]

Pin importlib-metadata<=4.13.0. Remove when 3.7 support dropped.

Fixes #211.

Changes

• nwm_client_new dependency, importlib-metadata pinned to <=4.13.0

Checklist

• [x] PR has an informative and human-readable title
• [x] PR is well outlined and documented. See #12 for an example
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
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• [x] Passes all existing automated tests
• [x] Any change in functionality is tested
• [x] New functions are documented (with a description, list of inputs, and expected output) using numpy docstring formatting
• [x] Placeholder code is flagged / future todos are captured in comments
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[aaraney]

@jarq6c, thanks!