Pin pyelftools<0.30 for Python 2 in docs/requirements.txt

Docs only installs fail under python2 due to trying to install pyelftools>=0.30 which dropped py2 support.

Fixes Gallopsled#2372

Author: peace-maker

docs/requirements.txt

@@ -8,7 +8,8 @@ isort
 mako>=1.0.0
 paramiko>=1.15.2
 pip>=6.0.8
-pyelftools>=0.2.3
+pyelftools>=0.29, <0.30; python_version<'3'
+pyelftools>=0.29; python_version>='3'
 pygments>=2.0
 pypandoc
 pyserial>=2.7