Port test suite from nose to pytest.

This just eliminates all errors on the tests collection. Elimination of
failures is left to the next commit.

tests/test_ec2/test_general.py

@@ -1,8 +1,7 @@
 from __future__ import unicode_literals
 
 # Ensure 'assert_raises' context manager support for Python 2.6
-import tests.backport_assert_raises
-from nose.tools import assert_raises
+import pytest
 
 import boto
 import boto3
@@ -25,7 +24,7 @@ def test_console_output():
 def test_console_output_without_instance():
     conn = boto.connect_ec2("the_key", "the_secret")
 
-    with assert_raises(EC2ResponseError) as cm:
+    with pytest.raises(EC2ResponseError) as cm:
         conn.get_console_output("i-1234abcd")
     cm.exception.code.should.equal("InvalidInstanceID.NotFound")
     cm.exception.status.should.equal(400)