mail/rss2email3: Unbreak

Port changes:

• Add previously missing manpage to PLIST_FILES
• Add USE_PYTHON=concurrent
• Add NO_ARCH=yes
• Group USExx
• Split out post-install-DOCS-on from post-install
• While here, update 3.8 -> 3.9 and change to DISTVERSION

Approved by: portmgr (port compliance, infrastructure)