dev-cmd/pr-automerge: exclude draft PRs

[bayandin]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?
• Have you successfully run brew man locally and committed any changes?

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This PR prevents draft PRs to be merged by pr-automerge.

By now, there are only 2 PR were automatically merged being a draft: Homebrew/homebrew-core#61327 and Homebrew/homebrew-core#64346 (see query).

I'm not sure if we need to add a flag to override this behaviour (as we have --without-approval).

[BrewTestBot]

Review period will end on 2020-12-16 at 12:34:16 UTC.

[BrewTestBot]

Review period ended.