Oh, what a pain, if you have to do it by hand! Is there no other way of
doing it by counting record lengths (in a column) - see below?

Just for the grins of it, I modified collections.html a bit to show more
on the first page. Then I printed a PDF copy of the online web pages. I
include that print as reference. 

When looking at the HTML source, I find the following lines at the
beginning of the relevant table:

<table border="1" class="docutils">
<colgroup>
<col width="21%" />
<col width="18%" />
<col width="18%" />
<col width="43%" />
</colgroup>

And the the first table line follows:
<thead valign="bottom">
<tr><th class="head">ABC</th>
<th class="head">Inherits</th>
<th class="head">Abstract Methods</th>
<th class="head">Mixin Methods</th>
</tr>
</thead>

So somebody is calculating the column width :) And this somebody - I
guess - is Sphinx? Is is possible to do the same for latex? Hmmm.