buildrpm: bump bundled Python version to 2.7.16 when building for centos{5,6}

When building rpm packages for centos 5 and 6, we bundle a mercurial-specific
version of Python 2.7 in /opt/python-hg.

This change is analogous to 5e947367606c, and bumps the embedded Python version
from 2.7.14 (released in 2017) to 2.7.16 (latest as of today).

contrib/Makefile.python

@@ -1,4 +1,4 b''
-PYTHONVER=2.7.14
+PYTHONVER=2.7.16
 PYTHONNAME=python-
 PREFIX=$(HOME)/bin/prefix-$(PYTHONNAME)$(PYTHONVER)
 SYMLINKDIR=$(HOME)/bin

contrib/packaging/buildrpm

@@ -20,8 +20,8 b' while [ "$1" ]; do'
         ;;
     --withpython | --with-python)
         shift
-        PYTHONVER=2.7.14
-        PYTHONMD5=cee2e4b33ad3750da77b2e85f2f8b724
+        PYTHONVER=2.7.16
+        PYTHONMD5=f1a2ace631068444831d01485466ece0
         ;;
     --rpmbuilddir )
         shift