Use pytest instead of nosetests

Nose has been in maintenance mode for the past several years
and pytest is a good replacement.

Other changes:
- Replace deprecated assertRegexpMatches with assertRegex
- Replace deprecated assertRaisesRegexp with assertRaisesRegex
- Replace deprecated SafeConfigParser with ConfigParser
- Force reinstall pytest and mock in tox virtualenv. This is because
  the globally installed packages may not work as expected(occured in
  jenkins job).

JIRA: RHELCMP-1619
Signed-off-by: Haibo Lin <hlin@redhat.com>

tests/test_extra_files_phase.py

@@ -244,9 +244,7 @@ class TestCopyFiles(helpers.PungiTestCase):
                 self.compose, [cfg], "x86_64", self.variant, package_sets, self.metadata
             )
 
-        self.assertRegexpMatches(
-            str(ctx.exception), r"No.*package.*matching bad-server\*.*"
-        )
+        self.assertRegex(str(ctx.exception), r"No.*package.*matching bad-server\*.*")
 
         self.assertEqual(len(get_file_from_scm.call_args_list), 0)
         self.assertEqual(get_dir_from_scm.call_args_list, [])