Auto-skip zope tests if zope.interface not present

This is a little more error-prone, but it's much more convenient to
invoke, since it doesn't involve skipping the "zope" marker.

tests/test_validators.py

@@ -30,8 +30,13 @@
 
 @pytest.fixture(scope="module")
 def zope():
-    import zope
-    import zope.interface
+    try:
+        import zope
+        import zope.interface
+    except ImportError:
+        raise pytest.skip(
+            "zope-related tests skipped when zope.interface is not installed"
+        )
 
     return zope
 
@@ -239,7 +244,6 @@ def f():
     return IFoo
 
 
-@pytest.mark.zope
 class TestProvides(object):
     """
     Tests for `provides`.

tox.ini

@@ -1,7 +1,6 @@
 [pytest]
 addopts = -ra
 testpaths = tests
-markers = zope
 filterwarnings =
     once::Warning
     ignore:::pympler[.*]