• update to 1.7.4

while here:

• replace PKGNAMEPREFIX with PYTHON_PKGNAMEPREFIX
• move pkg-plist contents to PLIST_FILES
• using of bsd.port.pre|post.mk isn't needed here

PR: 177580
Submitted by: Veniamin Gvozdikov <g.veanimin@googlemail.com> (maintainer)