Require Sphinx 2.1.0 and above

[SilverRainZ]

This extension use sphinx.transforms.post_transforms.SphinxPostTransform helper class, and it is introduced in Sphinx 2.1.0

ref: https://www.sphinx-doc.org/en/master/changes.html#release-2-1-0-released-jun-02-2019

[welcome[bot]]

Thanks for submitting your first pull request! You are awesome! :hugs:
If you haven't done so already, check out EBP's Code of Conduct and our Contributing Guide, as this will greatly help the review process.
Welcome to the EBP community! :tada:

[SilverRainZ]

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