Provide 'sys.flags.warn_default_encoding' for the tests to skip on Py…

…thon 3.7.

conftest.py

@@ -0,0 +1,23 @@
+import sys
+
+import pytest
+
+
+class Flags:
+    warn_default_encoding = 0
+
+    def __getattr__(self, *args):
+        return getattr(sys.flags, *args)
+
+
+@pytest.fixture(scope="session")
+def monkeysession():
+    with pytest.MonkeyPatch.context() as mp:
+        yield mp
+
+
+@pytest.fixture(scope="session", autouse=True)
+def future_flags(monkeysession):
+    if sys.version_info > (3, 8):
+        return
+    monkeysession.setattr(sys, 'flags', Flags())