www/py-grip: Fix runtime with py-markdown 3.4+

• Update version requirement of RUN_DEPENDS: Make sure py-markdown 3.4+ is used
• Bump PORTREVISION for package change

from [1]:
In addition, the md_globals parameter of
Markdown.extensions.Extension.extendMarkdown() is no longer recognized as a
valid parameter and will raise an error if provided.

Reference: https://github.com/Python-Markdown/markdown/blob/master/docs/changelog.md [1]