Fix class reference (#110)

Author: Gallaecio

cssselect/xpath.py

@@ -180,7 +180,7 @@ def css_to_xpath(self, css, prefix='descendant-or-self::'):
             This string is prepended to the XPath expression for each selector.
             The default makes selectors scoped to the context node’s subtree.
         :raises:
-            :class:`SelectorSyntaxError` on invalid selectors,
+            :class:`~cssselect.SelectorSyntaxError` on invalid selectors,
             :class:`ExpressionError` on unknown/unsupported selectors,
             including pseudo-elements.
         :returns:

tests/requirements.txt

@@ -1,5 +1,5 @@
 codecov
 lxml;python_version!="3.4"
 lxml<=4.3.5;python_version=="3.4"
-pytest
+pytest >=4.6, <4.7  # 4.7 drops support for Python 2.7 and 3.4
 pytest-cov