textproc/py-wtforms: update to 3.1.2

• Use ${DISTVERSION} over ${PORTVERSION}
• ${DISTNAME} is no longer needed
• Switch to Python PEP517 build
• Add EMAIL option for email validation in forms
• Build and install wtforms(1) man page (depends on Sphinx)
• Configure ${DOCS} build to use Sphinx for generating extensive project documentation
• Switch to py-tox for do-test

Changes:
https://github.com/wtforms/wtforms/blob/master/CHANGES.rst

PR: 275931
Submitted by: Alastair Hogge <agh@riseup.net>