Fix errors related to warnings raised by xdist

- pytester was creating a 'pexpect' directory to serve as temporary dir, but due to the fact that
   xdist adds the current directory to sys.path, that directory was being considered as candidate
   for import as a package. The directory is empty and a warning was being raised about
   it missing __init__ file, which is now turned into an error by our filterwarnings config
   in pytest.ini.

- Decided to play it safe and ignore any warnings during `pytest.importorskip`.

- pytest-xdist and execnet raise two warnings which should be fixed upstream:
   pytest-dev/pytest-xdist/issues/133

_pytest/pytester.py

@@ -1008,7 +1008,7 @@ class Testdir(object):
         The pexpect child is returned.
 
         """
-        basetemp = self.tmpdir.mkdir("pexpect")
+        basetemp = self.tmpdir.mkdir("temp-pexpect")
         invoke = " ".join(map(str, self._getpytestargs()))
         cmd = "%s --basetemp=%s %s" % (invoke, basetemp, string)
         return self.spawn(cmd, expect_timeout=expect_timeout)

_pytest/runner.py

@@ -554,14 +554,21 @@ def importorskip(modname, minversion=None):
     __version__ attribute.  If no minversion is specified the a skip
     is only triggered if the module can not be imported.
     """
+    import warnings
     __tracebackhide__ = True
     compile(modname, '', 'eval') # to catch syntaxerrors
     should_skip = False
-    try:
-        __import__(modname)
-    except ImportError:
-        # Do not raise chained exception here(#1485)
-        should_skip = True
+
+    with warnings.catch_warnings():
+        # make sure to ignore ImportWarnings that might happen because
+        # of existing directories with the same name we're trying to
+        # import but without a __init__.py file
+        warnings.simplefilter('ignore')
+        try:
+            __import__(modname)
+        except ImportError:
+            # Do not raise chained exception here(#1485)
+            should_skip = True
     if should_skip:
         raise Skipped("could not import %r" %(modname,), allow_module_level=True)
     mod = sys.modules[modname]

tox.ini

@@ -181,6 +181,10 @@ filterwarnings= error
                 ignore:bad escape.*:DeprecationWarning:re
                 # produced by path.readlines
                 ignore:.*U.*mode is deprecated:DeprecationWarning
+                # produced by pytest-xdist
+                ignore:.*type argument to addoption.*:DeprecationWarning
+                # produced by python >=3.5 on execnet (pytest-xdist)
+                ignore:.*inspect.getargspec.*deprecated, use inspect.signature.*:DeprecationWarning
 
 [flake8]
 ignore =E401,E225,E261,E128,E124,E301,E302,E121,E303,W391,E501,E231,E126,E701,E265,E241,E251,E226,E101,W191,E131,E203,E122,E123,E271,E712,E222,E127,E125,E221,W292,E111,E113,E293,E262,W293,E129,E702,E201,E272,E202,E704,E731,E402