Do not import directly into Retry tests (#1155)

The Retry test class was importing Repository directly, which had the
side effect of masking the tests.Repository import in AllTests with
github.Repository. Switch to importing github only, which is safe. As a
drive-by, switch to using self.assertRaises for one of the tests.

tests/Retry.py

@@ -32,8 +32,7 @@ import Framework
 
 import requests
 
-from github import GithubException
-from github.Repository import Repository
+import github
 
 REPO_NAME = 'PyGithub/PyGithub'
 
@@ -53,10 +52,9 @@ class Retry(Framework.TestCase):
         Framework.TestCase.setUp(self)
 
     def testShouldNotRetryWhenStatusNotOnList(self):
-        try:
+        with self.assertRaises(github.GithubException):
             self.g.get_repo(REPO_NAME)
-        except GithubException:
-            self.assertEquals(len(httpretty.latest_requests), 1)
+        self.assertEquals(len(httpretty.latest_requests), 1)
 
     def testReturnsRepoAfter3Retries(self):
         repository = self.g.get_repo(REPO_NAME)
@@ -64,7 +62,7 @@ class Retry(Framework.TestCase):
         for request in httpretty.latest_requests:
             self.assertEquals(request.path, '/repos/' + REPO_NAME)
 
-        self.assertIsInstance(repository, Repository)
+        self.assertIsInstance(repository, github.Repository.Repository)
         self.assertEquals(repository.full_name, REPO_NAME)
 
     def testReturnsRepoAfter1Retry(self):
@@ -73,7 +71,7 @@ class Retry(Framework.TestCase):
         for request in httpretty.latest_requests:
             self.assertEquals(request.path, '/repos/' + REPO_NAME)
 
-        self.assertIsInstance(repository, Repository)
+        self.assertIsInstance(repository, github.Repository.Repository)
         self.assertEquals(repository.full_name, REPO_NAME)
 
     def testRaisesRetryErrorAfterMaxRetries(self):