Fix test compatibility with ZODB 5.3.0

Open files can now be pickled under Python 2.

We had to switch to a custom object: open sockets can be pickled under
PyPy, which was the only other trivial stdlib example I could think
of. This way, at least, we're guaranteed it never happens again.

.travis.yml

@@ -5,12 +5,8 @@ python:
     - 3.4
     - 3.5
     - 3.6
-    # Force a newer PyPy on the old 'precise' CI image
-    # in order to install 'cryptography' needed for coveralls
-    # After September 2017 this should be the default and the version
-    # pin can be removed.
-    - pypy-5.6.0
-    - pypy3.5-5.8.0
+    - pypy
+    - pypy3
 install:
     - pip install -U pip setuptools
     - pip install -U coverage coveralls

src/zope/session/api.rst

@@ -98,14 +98,19 @@ Restrictions
 ------------
 
 Data stored in the session must be persistent or picklable.
+(Exactly which builtin and standard objects can be pickled depends on
+the Python version, the Python implementation, and the ZODB version,
+so we demonstrate with a custom object.)
 
     >>> import transaction
-    >>> with open(__file__) as f:
-    ...     session['oops'] = f
-    ...     transaction.commit()
+    >>> class NoPickle(object):
+    ...     def __reduce__(self):
+    ...          raise TypeError("I cannot be pickled")
+    >>> session['oops'] = NoPickle()
+    >>> transaction.commit()
     Traceback (most recent call last):
         [...]
-    TypeError: ...
+    TypeError: I cannot be pickled
 
 Clean up: