ntpsec: Add python313 variant.

Also removes redundant platforms line.

TESTED:
Built and ran tests with all variants on 10.4-10.5 ppc, 10.4-10.6
i386, 10.5-12.x x86_64, and 11.x-15.x arm64.  Successful except where
pythonXY was unavailable or broken on the OS/CPU combination.

sysutils/ntpsec/Portfile

@@ -12,7 +12,6 @@ categories          sysutils net
 maintainers         {fwright.net:fw @fhgwright} openmaintainer
 description         A secure, hardened, and improved implementation of NTP
 license             Permissive
-platforms           darwin
 long_description    A secure, hardened, and improved implementation of Network \
                     Time Protocol derived from NTP Classic, Dave Mills’s original.
 homepage            https://www.ntpsec.org/
@@ -28,7 +27,7 @@ checksums           rmd160  f6ee175289710afd8c053dd39a5a825d995c7a15 \
 # We skip 2.6 and 3.3, but keep 2.7 and 3.4+.
 # Some variants may force a more restricted list.
 #
-set pythons_suffixes {27 34 35 36 37 38 39 310 311 312}
+set pythons_suffixes {27 34 35 36 37 38 39 310 311 312 313}
 
 set pythons_ports {}
 foreach s ${pythons_suffixes} {