Shadow the built in tmpdir fixture to use our Path object

* Uses the same generation method as the built in tmpdir, simply
  returns a different object type.

tests/conftest.py

+import py
+import pytest
+
+from tests.lib.path import Path
+
+
+@pytest.fixture
+def tmpdir(request):
+    """
+    Return a temporary directory path object which is unique to each test
+    function invocation, created as a sub directory of the base temporary
+    directory. The returned object is a ``tests.lib.path.Path`` object.
+
+    This is taken from pytest itself but modified to return our typical
+    path object instead of py.path.local.
+    """
+    name = request.node.name
+    name = py.std.re.sub("[\W]", "_", name)
+    tmp = request.config._tmpdirhandler.mktemp(name, numbered=True)
+    return Path(tmp)