[fix] qwant engine: set header Accept-Language to bypass bot detection (#5382)

Set HTTP header Accept-Language [1] for the Qwant engine.

Qwant does not seem to work on any SearXNG instance right now, and this is a fix
for this issue.

During testing, it seems like setting the Accept-Language gives more success for
bypassing bot detection (tested with a few ~20 searches).

[1] https://docs.searxng.org/dev/engines/enginelib.html#searx.enginelib.Engine.send_accept_language_header

searx/engines/qwant.py

@@ -82,6 +82,9 @@ max_page = 5
 """5 pages maximum (``&p=5``): Trying to do more just results in an improper
 redirect"""
 
+# Otherwise Qwant will return 403 if not set
+send_accept_language_header = True
+
 qwant_categ = None
 """One of ``web-lite`` (or ``web``), ``news``, ``images`` or ``videos``"""