www/py-flask-smorest: Update to 0.26.0

Update with backwards-incompatible changes:

• Use warnings.warn rather than log warnings in application log
• Support webargs 7. Drop support for webargs 6

Other changes:

• Official Python 3.9 support
• Bound dependencies versions in setup.py

Changelog: https://github.com/marshmallow-code/flask-smorest/blob/master/CHANGELOG.rst#0260-2020-12-17

PR: 252489
Submitted by: Goran Mekic
Approved by: arrowd, tcberner (mentors, implicit)