v0.3.1

This release carries no changes to the library's behaviour. It updates project
maturity, documentation, and packaging metadata.

Changed

• Development status is now Beta. The public API (the curved_text function and
  the CurvedText class, with pos, anchor, offset, box, and the keyword
  pass-through) has been stable across releases.

Added

• Hosted documentation and API reference at
  thiebes.github.io/curved-text.
• A docs extra (pip install curved-text[docs]) for building the
  documentation locally.