bpo-32924: Fix the Show Source url in the docs. (GH-5835)

The Show Source link in Python 3.7 docs is pointing to GitHub's master branch.
It should point to the 3.7 branch.

Doc/tools/templates/customsourcelink.html

@@ -4,7 +4,7 @@
     <ul class="this-page-menu">
       <li><a href="{{ pathto('bugs') }}">{% trans %}Report a Bug{% endtrans %}</a></li>
       <li>
-        <a href="https://github.com/python/cpython/blob/master/Doc/{{ sourcename|replace('.rst.txt', '.rst') }}"
+        <a href="https://github.com/python/cpython/blob/{{ version }}/Doc/{{ sourcename|replace('.rst.txt', '.rst') }}"
             rel="nofollow">{{ _('Show Source') }}
         </a>
       </li>