update mkdocs to be compatible with python 3.7+

fix #261

yeti/content.html

@@ -1,20 +1,20 @@
-{% if meta.source %}
+{% if page.meta.source %}
 <div class="source-links">
-{% for filename in meta.source %}
+{% for filename in page.meta.source %}
     <span class="label label-primary">{{ filename }}</span>
 {% endfor %}
 </div>
 {% endif %}
 
-{{ content }}
+{{ page.content }}
 
 <div id="disqus_thread"></div>
 <script>
     (function() {
         var d = document, s = d.createElement('script');
-        
+
         s.src = '//learnopengl-cn.disqus.com/embed.js';
-        
+
         s.setAttribute('data-timestamp', +new Date());
         (d.head || d.body).appendChild(s);
     })();