Drop support to old pytest versions and test with python 3.7

testing/acceptance_test.py

@@ -708,16 +708,6 @@ def test_sub_plugins_disabled(testdir, plugin):
 
 
 class TestWarnings:
-    @pytest.fixture(autouse=True)
-    def skip_if_unsupported_pytest_version(self):
-        """Skip tests of this class if we are running in a pytest version which does not
-        support warnings yet.
-        """
-        from pkg_resources import parse_version
-
-        if parse_version(pytest.__version__) < parse_version("3.1"):
-            pytest.skip("pytest warnings requires >= 3.1")
-
     @pytest.mark.parametrize("n", ["-n0", "-n1"])
     @pytest.mark.parametrize("warn_type", ["pytest", "builtin"])
     def test_warnings(self, testdir, n, warn_type):

testing/test_looponfail.py

@@ -1,4 +1,7 @@
 import py
+import pytest
+from pkg_resources import parse_version
+
 from xdist.looponfail import RemoteControl
 from xdist.looponfail import StatRecorder
 
@@ -214,7 +217,11 @@ class TestLooponFailing:
         assert "test_one" not in remotecontrol.failures[0]
         assert "test_two" in remotecontrol.failures[0]
 
-    @py.test.mark.xfail(py.test.__version__ >= "3.1", reason="broken by pytest 3.1+")
+    @pytest.mark.xfail(
+        parse_version(pytest.__version__) >= parse_version("3.1"),
+        reason="broken by pytest 3.1+",
+        strict=True,
+    )
     def test_looponfail_removed_test(self, testdir):
         modcol = testdir.getmodulecol(
             """

testing/test_remote.py

@@ -1,7 +1,6 @@
 import py
 import pprint
 import pytest
-from pkg_resources import parse_version
 
 from xdist.workermanage import WorkerController, unserialize_report
 from xdist.remote import serialize_report
@@ -294,10 +293,6 @@ class TestWorkerInteractor:
         ev = worker.popevent("workerfinished")
         assert "workeroutput" in ev.kwargs
 
-    @pytest.mark.skipif(
-        parse_version(pytest.__version__) < parse_version("3.3"),
-        reason="skip at module level illegal in this pytest version",
-    )
     def test_remote_collect_skip(self, worker):
         worker.testdir.makepyfile(
             """