v8.6.0

Significant improvements:

• #384 via PR #385, PR #406: Exposed type stubs with
  annotations for public API -- by @kasium.

• PR #401 (related to the PR #352 effort): Started reusing the
  the expriration_interval setting in the low-level
  select.select() invocation,
  effectively reducing the system
  load under the Windows OS when idle, that is noticeable on low-end
  hardware systems -- by :user:MichaIng.

Internal changes:

• Implemented a manual-trigger-based release workflow.
• Integrated publishing GitHub Releases into the workflow.
• Migrated the docs theme to Furo
  (created by @pradyunsg).
• Attempted to improve the stability of testing.
• Configured the CI to test the same distribution as will be shipped.
• Improved the linting setup and contributor checklists.
• Stopped running tests under Ubuntu 16.04.
• Tweaked the distribution packages metadata to satisfy strict checks.
• Implemented distribution build reproducibility using a pip constraints
  lock-file.
• Added per-environment lock-files into the tox test environments.