Move TWMock class to a fixture

Using a relative import like before was not very nice

testing/code/test_code.py

@@ -1,8 +1,6 @@
 import sys
 from unittest import mock
 
-from test_excinfo import TWMock
-
 import _pytest._code
 import pytest
 
@@ -168,17 +166,15 @@ class TestTracebackEntry:
 
 
 class TestReprFuncArgs:
-    def test_not_raise_exception_with_mixed_encoding(self):
+    def test_not_raise_exception_with_mixed_encoding(self, tw_mock):
         from _pytest._code.code import ReprFuncArgs
 
-        tw = TWMock()
-
         args = [("unicode_string", "São Paulo"), ("utf8_string", b"S\xc3\xa3o Paulo")]
 
         r = ReprFuncArgs(args)
-        r.toterminal(tw)
+        r.toterminal(tw_mock)
 
         assert (
-            tw.lines[0]
+            tw_mock.lines[0]
             == r"unicode_string = São Paulo, utf8_string = b'S\xc3\xa3o Paulo'"
         )