Specify minimum version for mistletoe

Previous versions of this package can cause the documentation build to
abort when parsing, e.g., vendored-in docs such as the one of the
debug module.

python-requirements.txt

diff --git a/python-requirements.txt b/python-requirements.txt
index 7ccf2a2..51464ea 100644
--- a/python-requirements.txt
+++ b/python-requirements.txt
@@ -1,5 +1,5 @@
 livereload
-mistletoe
+mistletoe>=0.7.2
 hjson
 pygments
 mako