Fix nondeterministic test failures in tales.rst

The test does the following:

  - write a page template to a file on disk in a temporary directory
  - create a view class with a ViewPageTemplateFile() attribute
  - rewrite the same page template file
  - expect the ViewPageTemplateFile() to notice the changed mtime and
    reload the template
  - (and it does the rewrite + expect a second time later)

This can easily fail when the writes happen fast enough that the
temporary file gets the same mtime value on disk, depending on
filesystem timestamp granularity.

To avoid the failure we can trick the ViewPageTemplateFile into thinking
that the template was never loaded so it will definitely be reloaded.

I don't particularly like this solution: it depends on the
implementation details of SimpleViewClass, ViewPageTemplateFile and
BoundPageTemplate too much.

Closes #12.

docs/tales.rst

@@ -199,6 +199,10 @@ We are now updating our original template to provide the message text:
   ... </html>
   ... ''')
 
+Let's make sure the template will be reloaded from disk
+
+  >>> FrontPage.index.__func__._v_last_read = 0
+
 Now we should get two message boxes with different text:
 
   >>> print(view().strip())
@@ -260,6 +264,7 @@ well:
   ...   </body>
   ... </html>
   ... ''')
+  >>> FrontPage.index.__func__._v_last_read = 0
 
 Now we should get two message boxes with different text and types: