Fix 2 DeprecationWarnings about invalid escape seq

src/_pytest/python_api.py

@@ -497,7 +497,7 @@ def _is_numpy_array(obj):
 
 
 def raises(expected_exception, *args, **kwargs):
-    """
+    r"""
     Assert that a code block/function call raises ``expected_exception``
     and raise a failure exception otherwise.
 

src/_pytest/recwarn.py

@@ -85,7 +85,7 @@ class _DeprecatedCallContext(object):
 
 
 def warns(expected_warning, *args, **kwargs):
-    """Assert that code raises a particular class of warning.
+    r"""Assert that code raises a particular class of warning.
 
     Specifically, the parameter ``expected_warning`` can be a warning class or
     sequence of warning classes, and the inside the ``with`` block must issue a warning of that class or