Fix usage of deprecated unittest API.

See #2319.

runtests.py

@@ -1583,7 +1583,7 @@ class EmbedTest(unittest.TestCase):
         if sys.version_info[0] >=3 and CY3_DIR:
             cython = os.path.join(CY3_DIR, cython)
         cython = os.path.abspath(os.path.join('..', '..', cython))
-        self.assert_(os.system(
+        self.assertTrue(os.system(
             "make PYTHON='%s' CYTHON='%s' LIBDIR1='%s' test > make.output" % (sys.executable, cython, libdir)) == 0)
         try:
             os.remove('make.output')