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>