xkcd: switch to DDG for keyword lookup

[dgw]

Description

Bing has become supremely unreliable. In a shocking turn of events, it seems that DuckDuckGo no longer vomits on the long string of -site: operators used in this plugin.

Current master:

16:03:35 <~dgw> ,xkcd group chat
16:03:36 <SopelTest> dgw: Could not find any comics for that query.

# [Ed. note: Same for any other non-integer query]

With this patch:

16:31:03 <~dgw> ,xkcd group chat
16:31:04 <SopelTest> [xkcd] Group Chat Rules | Alt-text: There's no group chat member more enigmatic than the cool
                     person who you all assume has the chat on mute, but who then instantly chimes in with no delay
                     the moment something relevant to them is mentioned. | https://xkcd.com/2235

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches (see before/after above)

Notes

Someone else can investigate backporting this to 7.1.x if they like, but it's easy enough for someone to just use the newer xkcd.py in an old bot if they want. The sopel.modules.search stuff hasn't really changed. I don't particularly want to spend time on things that don't progress 8.0 at this point.

This is the simplest, fastest way to get .xkcd some keywords working again, because .ddg output works as expected. I might still try to fix Bing search separately… Had some small amount of success by fussing with the user agent Sopel sends, but it still wouldn't reliably obey site: operators. (It's probably picking up ads on the page; I would bet the HTML has changed.)

[SnoopJ]

Change looks fine to me since it's "just" a different configuration, and you've tested it. Especially if Bing is outright failing that often