Use -n auto now that xdist behaves well in Travis and AppVeyor

This hopefully fixes the flaky test_idval_hypothesis on AppVeyor

Fix #3707

Author: nicoddemus

testing/python/metafunc.py

@@ -211,9 +211,6 @@ class A(object):
     @hypothesis.settings(
         deadline=400.0
     )  # very close to std deadline and CI boxes are not reliable in CPU power
-    @pytest.mark.xfail(
-        sys.platform.startswith("win32"), reason="flaky #3707", strict=False
-    )
     def test_idval_hypothesis(self, value):
         from _pytest.python import _idval
 

tox.ini

@@ -40,7 +40,7 @@ deps =
     mock
     nose
 commands =
-    pytest -n3 -ra --runpytest=subprocess {posargs:testing}
+    pytest -n auto -ra --runpytest=subprocess {posargs:testing}
 
 
 [testenv:linting]
@@ -58,7 +58,7 @@ deps =
     {env:_PYTEST_TOX_EXTRA_DEP:}
 changedir=testing
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n8 -ra {posargs:.}
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto -ra {posargs:.}
 
 [testenv:py36-xdist]
 deps = {[testenv:py27-xdist]deps}
@@ -91,7 +91,7 @@ setenv =
     {[testenv]setenv}
     PYTHONDONTWRITEBYTECODE=1
 commands =
-    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n3 -ra {posargs:.}
+    {env:_PYTEST_TOX_COVERAGE_RUN:} pytest -n auto -ra {posargs:.}
 
 [testenv:py27-trial]
 deps =