Merge remote-tracking branch 'upstream/master' into features

doc/en/example/simple.rst

@@ -269,6 +269,7 @@ running from a test you can do something like this:
         sys._called_from_test = True
 
     def pytest_unconfigure(config):
+        import sys
         del sys._called_from_test
 
 and then check for the ``sys._called_from_test`` flag:

doc/en/talks.rst

@@ -2,6 +2,7 @@
 Talks and Tutorials
 ==========================
 
+..
    .. sidebar:: Next Open Trainings
 
       `Professional Testing with Python
@@ -10,6 +11,12 @@ Talks and Tutorials
 
 .. _`funcargs`: funcargs.html
 
+Books
+---------------------------------------------
+
+- `Python Testing with pytest, by Brian Okken (2017)
+  <https://pragprog.com/book/bopytest/python-testing-with-pytest>`_.
+
 Talks and blog postings
 ---------------------------------------------