Fixed all pep8 warnings

templates/header.html

@@ -5,7 +5,7 @@
             <input type="text" name="search" placeholder="Find your favourite products" class="search-bar">
             <input type="submit" class="search-button">
         </form>
-        {% if user != None: %}
+        {% if user is not None: %}
             <a href="/logout">Welcome, {{ user.username }}</a>
         {% else %}
             <a href="/login">Login/Signup</a>
@@ -13,7 +13,7 @@
     </nav>
     
     <centre>
-        {% if user != None and user.role == "Seller" %}
+        {% if user is not None and user.role == "Seller" %}
             <div class="categories">
                 {# List all available seller tools #}
                 <a href="/products/add" class="category">Create Products</a>