perf testsuite: python 3 fixes

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.