build: eliminate use of python execfile command

As discussed in
id:8cc9dd580ad672527e12f43706f9803b2c8e99d8.1405220724.git.wking@tremily.us,
execfile is unavailable in python3.

The approach of this commit avoids modifying the python module path,
which is arguably preferable since it avoids potentially accidentally
importing a module from the wrong place.

devel/release-checks.sh

@@ -130,7 +130,7 @@ else
 fi
 
 echo -n "Checking that python bindings version is $VERSION... "
-py_version=`python -c "execfile('$PV_FILE'); print __VERSION__"`
+py_version=`python -c "with open('$PV_FILE') as vf: exec(vf.read()); print __VERSION__"`
 if [ "$py_version" = "$VERSION" ]
 then
 	echo Yes.

doc/prerst2man.py

@@ -10,7 +10,8 @@ outdir = argv[2]
 if not isdir(outdir):
     makedirs(outdir, 0o755)
 
-execfile(sourcedir + "/conf.py")
+with open(sourcedir + "/conf.py") as cf:
+    exec(cf.read())
 
 
 def header(file, startdocname, command, description, authors, section):