Security fix documentation dependencies (#342)

According to dependabot Pygment versions < 2.7.4 are vulnerable.
Updating to this version even though it should not be a problem with our static documentation.

CHANGES.rst

@@ -11,6 +11,9 @@
 - Rework ``fsrefs`` script to work significantly faster by optimizing how it does
   IO. See `PR 340 <https://github.com/zopefoundation/ZODB/pull/340>`_.
 
+- Require Python 3 to build the documentation.
+
+
 5.6.0 (2020-06-11)
 ==================
 

doc/requirements.txt

 Sphinx
-# pygments 2.6 stops the support for python2
-pygments<2.6
+# Silence dependabot claiming a security issue in older versions:
+pygments >= 2.7.4
 docutils
 ZODB
 sphinxcontrib_zopeext