Log Message:
textproc/itstool: Declare python 2.7 not acceptable

With python 2.7, itstool fails to process the cs translation as part
of the gnumeric112 build.  This appears to be about confusion between
ascii and utf-8 on stderr or something like that.  Because it appears
that few who might debug this are still using python 2.7, mark it
incompatible, which will cause the python3.7 version of itstool to be
built and used.

Proposed on pkgsrc-users without objection, and with no reports of the
2.7 version working.