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_createrepophase.py

@@ -1341,7 +1341,7 @@ class TestGetProductIds(PungiTestCase):
             get_productids_from_scm(self.compose)
 
         self.assertEqual(get_dir_from_scm.call_args_list, [mock.call(cfg, mock.ANY)])
-        self.assertRegexpMatches(
+        self.assertRegex(
             str(ctx.exception),
             r"No product certificate found \(arch: amd64, variant: (Everything|Client)\)",  # noqa: E501
         )
@@ -1365,6 +1365,4 @@ class TestGetProductIds(PungiTestCase):
             get_productids_from_scm(self.compose)
 
         self.assertEqual(get_dir_from_scm.call_args_list, [mock.call(cfg, mock.ANY)])
-        self.assertRegexpMatches(
-            str(ctx.exception), "Multiple product certificates found.+"
-        )
+        self.assertRegex(str(ctx.exception), "Multiple product certificates found.+")