TST: update default junit_family

pytest 6 will be moving to this by default and azure supports this
format so might as well move to the future now.

This causes exceptions from pytest if we try to set it in conftest.py
with the markers

lib/matplotlib/testing/conftest.py

@@ -7,8 +7,8 @@
 def pytest_configure(config):
     # config is initialized here rather than in pytest.ini so that `pytest
     # --pyargs matplotlib` (which would not find pytest.ini) works.  The only
-    # entries in pytest.ini set minversion (which is checked earlier) and
-    # testpaths/python_files, as they are required to properly find the tests.
+    # entries in pytest.ini set minversion (which is checked earlier),
+    # testpaths/python_files, as they are required to properly find the tests
     for key, value in [
         ("markers", "flaky: (Provided by pytest-rerunfailures.)"),
         ("markers", "timeout: (Provided by pytest-timeout.)"),

pytest.ini

@@ -4,3 +4,4 @@ minversion = 3.6
 
 testpaths = lib
 python_files = test_*.py
+junit_family = xunit2