Skip posix path traversal test rows on a Windows filesystem

The parametrized traversal tests added in 1.4.2 run every spelling of a
traversal URI against both posixpath and ntpath, forcing os.path to
each in turn.  The ntpath half works on any host, as ntpath accepts
forward slashes, but the posixpath half cannot work on Windows:
TemplateLookup.get_template() converts the configured directory with
dir_.replace(os.path.sep, posixpath.sep), so with os.path forced to
posixpath the separator sought is "/" and the directory's real
backslashes are left in place.  The joined path then normalizes to
something that does not exist, and get_template() raises
TopLevelLookupException before the traversal check is ever consulted,
failing eleven cases on the Windows jobs.

The posixpath rows are now skipped when os.name is "nt".  Coverage is
not lost: those rows continue to run on the Linux and macOS jobs, and
on Windows the ntpath rows exercise the check against the real
os.path rather than an emulation of it.

Only the test suite was affected.  The traversal check itself is
unchanged.

Fixes: #441
Change-Id: I4cbb5f1d1793aecd1aafd288d771de596da4b88b

Author: zzzeek

doc/build/unreleased/441.rst

@@ -0,0 +1,11 @@
+.. change::
+    :tags: bug, tests
+    :tickets: 441
+
+    Fixed regression caused in 1.4.2 where tests added to the suite were
+    unable to run directly on Windows, due to posix mechanics: the tests force
+    ``os.path`` to ``posixpath``, whereas
+    :meth:`.TemplateLookup.get_template` converts the configured directory
+    using ``os.path.sep``.  These tests are now skipped on that platform,
+    where the traversal check is instead exercised against ``ntpath``
+    natively.

test/test_lookup.py

@@ -134,7 +134,23 @@ def test_dont_accept_relative_outside_of_root(self):
         runtime._lookup_template(ctx, "foo/../index.html", index.uri)
 
     @pytest.mark.parametrize(
-        "ospath", [posixpath, ntpath], ids=["posix", "windows"]
+        "ospath",
+        [
+            # get_template() converts the configured directory using
+            # os.path.sep; forcing posixpath on a Windows host would
+            # leave the directory's real backslashes in place and the
+            # template would not be located at all, so this half of the
+            # matrix only applies where the filesystem is posix
+            pytest.param(
+                posixpath,
+                marks=pytest.mark.skipif(
+                    os.name == "nt",
+                    reason="posix path semantics need a posix filesystem",
+                ),
+                id="posix",
+            ),
+            pytest.param(ntpath, id="windows"),
+        ],
     )
     @pytest.mark.parametrize(
         "uri",