Updates, candidate 0.9.1 release

[jgriffiths]

• PSBT and tx fixes and new helpers.
• Docs updates, add readthedocs config for the upcoming mandatory changes
• Bump version for a new release

[LeoComandini]

utACK 58595e887e04cc442eccfa5bf15c6585b66903b3 with minor nit, code review