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_livemediaphase.py

@@ -363,7 +363,7 @@ class TestLiveMediaPhase(PungiTestCase):
 
         phase = LiveMediaPhase(compose)
 
-        with self.assertRaisesRegexp(
+        with self.assertRaisesRegex(
             RuntimeError, r"no.+Missing.+when building.+Server"
         ):
             phase.run()
@@ -393,7 +393,7 @@ class TestLiveMediaPhase(PungiTestCase):
 
         phase = LiveMediaPhase(compose)
 
-        with self.assertRaisesRegexp(
+        with self.assertRaisesRegex(
             RuntimeError, r"There is no variant Missing to get repo from."
         ):
             phase.run()