fail doc generation if pygments is not installed

--HG--
branch : trunk

ISSUES.txt

@@ -107,3 +107,4 @@ Also consider implementing py.test --funcargs to
 show available funcargs - it should honour the 
 path::TestClass syntax so one can easily inspect
 where funcargs come from or which are available. 
+

doc/conftest.py

@@ -3,3 +3,7 @@ import py
 #py.test.importorskip("pygments")
 pytest_plugins = ['pytest_restdoc']
 collect_ignore = ['test/attic.txt']
+
+def pytest_runtest_setup(item):
+    if item.fspath.ext == ".txt":
+        import pygments # for raising an error