Replace the doc theme with the pydoctheme

Released under the Python Software Foundation License, this one
doesn't look awful (no green top bar, huzzah!).

Lightly forked it to remove the donation footer (we don't take any)
and to change the python references to fish references.

The image is just our favicon, which is a stylized "f" and therefore
not fantastic (are we facebook?), but it's the best I found, and the
thing before had no images at all.

Fixes #6500
(as far as I'm concerned)

doc_src/python_docs_theme/layout.html

@@ -0,0 +1,58 @@
+{% extends "classic/layout.html" %}
+
+{% block rootrellink %}
+    <li><img src="{{ pathto('_static/' + theme_root_icon, 1) }}" alt=""
+             style="vertical-align: middle; margin-top: -1px"/></li>
+    <li><a href="{{theme_root_url}}">{{theme_root_name}}</a>{{ reldelim1 }}</li>
+    {% if theme_root_include_title %}
+    <a href="{{ pathto('index') }}">{{ shorttitle }}</a>{{ reldelim1 }}
+    {% endif %}
+{% endblock %}
+
+{%- macro searchbox() %}
+{# modified from sphinx/themes/basic/searchbox.html #}
+    {%- if builder != "htmlhelp" %}
+    <div class="inline-search" style="display: none" role="search">
+        <form class="inline-search" action="{{ pathto('search') }}" method="get">
+          <input placeholder="{{ _('Quick search') }}" type="text" name="q" />
+          <input type="submit" value="{{ _('Go') }}" />
+          <input type="hidden" name="check_keywords" value="yes" />
+          <input type="hidden" name="area" value="default" />
+        </form>
+    </div>
+    <script type="text/javascript">$('.inline-search').show(0);</script>
+    {%- endif %}
+{%- endmacro %}
+
+{% block relbar1 %} {% if builder != 'qthelp' %} {{ relbar() }} {% endif %} {% endblock %}
+{% block relbar2 %} {% if builder != 'qthelp' %} {{ relbar() }} {% endif %} {% endblock %}
+{% block relbaritems %}
+    {%- if pagename != "search" and builder != "singlehtml" and builder != "htmlhelp" %}
+    <li class="right">
+        {{ searchbox() }}
+        {{ reldelim2 }}
+    </li>
+    {%- endif %}
+{% endblock %}
+
+{% block extrahead %}
+    <link rel="shortcut icon" type="image/png" href="{{ pathto('_static/' + theme_root_icon, 1) }}" />
+    {% if builder != "htmlhelp" %}
+    {% if not embedded %}<script type="text/javascript" src="{{ pathto('_static/copybutton.js', 1) }}"></script>{% endif %}
+    {% endif %}
+    {{ super() }}
+{% endblock %}
+
+{% block footer %}
+    <div class="footer">
+    &copy; <a href="{{ pathto('copyright') }}">{% trans %}Copyright{% endtrans %}</a> {{ copyright|e }}.
+    <br />
+    {%- if last_updated %}
+      {% trans last_updated=last_updated|e %}Last updated on {{ last_updated }}.{% endtrans %}
+    {%- endif %}
+    {% if theme_issues_url %}{% trans pathto_bugs=theme_issues_url %}<a href="{{ theme_issues_url }}">Found a bug</a>?{% endtrans %}{% endif %}
+    <br />
+
+    {% trans sphinx_version=sphinx_version|e %}Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> {{ sphinx_version }}.{% endtrans %}
+    </div>
+{% endblock %}