Add new difficulty: laike9m#12

Author: laike9m

docs/Contribute.md

@@ -2,9 +2,9 @@
 
 1. Fork this project if you haven't done so
 
-2. Determine how difficult the challenge is. There are 3 levels: **basic**, **intermediate**, and **advanced**.
+2. Determine how difficult the challenge is. There are 4 levels: **basic**, **intermediate**, **advanced**, and **extreme**.
 
-3. Create a new directory under [`challenges/`](https://github.com/laike9m/Python-Type-Challenges/tree/main/challenges). The new directory's name should follow the pattern `[basic|intermediate|advanced]-name`.
+3. Create a new directory under [`challenges/`](https://github.com/laike9m/Python-Type-Challenges/tree/main/challenges). The new directory's name should follow the pattern `[basic|intermediate|advanced|extreme]-name`.
 
    For example, say you want to add a new challenge about [Protocols](https://mypy.readthedocs.io/en/stable/protocols.html). Since this is an advanced topic, you may name the directory `advanced-protocol`.
 

templates/challenge_list.html

@@ -6,17 +6,26 @@
     "basic") %}
     <li><a href="/challenges/{{ challenge.name }}">{{ challenge.name }}</a></li>
     {% endfor %}
+
     <br />
     <b>Intermediate</b>
     {% for challenge in challenge_names | selectattr("difficulty", "equalto",
     "intermediate") %}
     <li><a href="/challenges/{{ challenge.name }}">{{ challenge.name }}</a></li>
     {% endfor %}
+
     <br />
     <b>Advanced</b>
     {% for challenge in challenge_names | selectattr("difficulty", "equalto",
     "advanced") %}
     <li><a href="/challenges/{{ challenge.name }}">{{ challenge.name }}</a></li>
     {% endfor %}
+
+    <br />
+    <b>Extreme</b>
+    {% for challenge in challenge_names | selectattr("difficulty", "equalto",
+    "extreme") %}
+    <li><a href="/challenges/{{ challenge.name }}">{{ challenge.name }}</a></li>
+    {% endfor %}
   </ol>
 </details>