Added search.html to epub_exclude_files

Added search.html to epub_exclude_files to avoid these errors:

Epubcheck Version 3.0

Validating against EPUB version 2.0
ERROR: pythonguide.epub/search.html(17,57): attribute "id" not allowed here; expected attribute "charset", "defer", "src" or "xml:space"
ERROR: pythonguide.epub/search.html(51,32): element "form" not allowed anywhere; expected the element end-tag, text or element "a", "abbr", "acronym", "address", "applet", "b", "bdo", "big", "blockquote", "br", "cite", "code", "del", "dfn", "div", "dl", "em", "h1", "h2", "h3", "h4", "h5", "h6", "hr", "i", "iframe", "img", "ins", "kbd", "map", "noscript", "ns:svg", "object", "ol", "p", "pre", "q", "samp", "script", "small", "span", "strong", "sub", "sup", "table", "tt", "ul" or "var" (with xmlns:ns="http://www.w3.org/2000/svg")
ERROR: pythonguide.epub/search.html(54,66): element "form" not allowed anywhere; expected the element end-tag, text or element "a", "abbr", "acronym", "address", "applet", "b", "bdo", "big", "blockquote", "br", "cite", "code", "del", "dfn", "div", "dl", "em", "h1", "h2", "h3", "h4", "h5", "h6", "hr", "i", "iframe", "img", "ins", "kbd", "map", "noscript", "ns:svg", "object", "ol", "p", "pre", "q", "samp", "script", "small", "span", "strong", "sub", "sup", "table", "tt", "ul" or "var" (with xmlns:ns="http://www.w3.org/2000/svg")

It should be noted that ePub does not use javascript for anything (or any scripting), so all those other pages with the quick search function do bad things to ePub files.  I'll send through the rest of the errors in a separate issue.