perf testsuite: python 3 fixes

There are a few errors when trying to run the performance testsuite with
Python 3.  This commit fixes them.

In Python 2, it was possible to use relative imports (importing a module
relative to the current one).  In Python 3 it isn't.  So I use
absolute_import from the __future__ module, which allows Python 2 to
behave like Python 3, and use the Python 3 syntax.

In Python 3, dict.iterkeys doesn't exist anymore.  Using dict.keys is a
good compromise in this case.

gdb/testsuite/ChangeLog:

	* gdb.perf/lib/perftest/perftest.py: Change relative imports to
	absolute.
	(SingleStatisticTestResult.report): Use dict.keys instead of
	dict.iterkeys.

gdb/testsuite/ChangeLog

@@ -1,3 +1,10 @@
+2016-01-08  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* gdb.perf/lib/perftest/perftest.py: Change relative imports to
+	absolute.
+	(SingleStatisticTestResult.report): Use dict.keys instead of
+	dict.iterkeys.
+
 2016-01-06  Pedro Alves  <palves@redhat.com>
 
 	* gdb.python/py-infthread.exp: Fix typo.  Expect t0.num to be 1.

gdb/testsuite/gdb.perf/lib/perftest/perftest.py

@@ -13,12 +13,15 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-import testresult
-import reporter
-from measure import Measure
-from measure import MeasurementCpuTime
-from measure import MeasurementWallTime
-from measure import MeasurementVmSize
+from __future__ import absolute_import
+
+import perftest.testresult as testresult
+import perftest.reporter as reporter
+from perftest.measure import Measure
+from perftest.measure import MeasurementCpuTime
+from perftest.measure import MeasurementWallTime
+from perftest.measure import MeasurementVmSize
+
 
 class TestCase(object):
     """Base class of all performance testing cases.

gdb/testsuite/gdb.perf/lib/perftest/testresult.py

@@ -42,7 +42,7 @@ class SingleStatisticTestResult(TestResult):
 
     def report(self, reporter, name):
         reporter.start()
-        for key in sorted(self.results.iterkeys()):
+        for key in sorted(self.results.keys()):
             reporter.report(name, key, self.results[key])
         reporter.end()