textproc/py-mistune: Update to 0.7.1

• Update PORTVERSION and distinfo checksum (0.7.1) [1]
• Update test target to new convention
• Add NO_ARCH

Changes:

https://github.com/lepture/mistune/blob/v0.7.1/CHANGES.rst

PR: 205516 [1]
Submitted by: Yuri <yuri rawbw com> [1]