Add PathComparer for testing and fix 2 test failure on Windows. (#5943)

commits are squashed.

* Add PathComparer for testing and fix 2 test failure on Windows.
* fix flake8
* add type information by f2f review. Thanks to @tk0miya!
* fix mypy, flake8 again..

tests/conftest.py

@@ -14,6 +14,7 @@ import pytest
 
 import sphinx
 from sphinx.testing.path import path
+from sphinx.testing import comparer
 
 pytest_plugins = 'sphinx.testing.fixtures'
 
@@ -47,3 +48,7 @@ def _initialize_test_directory(session):
 
 def pytest_sessionstart(session):
     _initialize_test_directory(session)
+
+
+def pytest_assertrepr_compare(op, left, right):
+    comparer.pytest_assertrepr_compare(op, left, right)