Move argv_handler fixture to conftest.

Use conftest module for fixtures we need in several test modules.

tests/conftest.py

+import pytest
+import sys
+
+
+@pytest.fixture(scope="function")
+def argv_handler(request):
+    """This fixture restores sys.argv and sys.stdin after tests.
+    """
+    _argv_stored = sys.argv
+    _stdin_stored = sys.stdin
+
+    def teardown():
+        sys.argv = _argv_stored
+        sys.stdin = _stdin_stored
+    request.addfinalizer(teardown)

tests/test_diceware.py

@@ -22,19 +22,6 @@ class FakeRandom(object):
         return elems[num]
 
 
-@pytest.fixture(scope="function")
-def argv_handler(request):
-    """This fixture restores sys.argv and sys.stdin after tests.
-    """
-    _argv_stored = sys.argv
-    _stdin_stored = sys.stdin
-
-    def teardown():
-        sys.argv = _argv_stored
-        sys.stdin = _stdin_stored
-    request.addfinalizer(teardown)
-
-
 class TestDicewareModule(object):
 
     def test_get_random_sources(self):