[fix] debug log: UnicodeEncodeError: 'ascii' codec can't encode

The issue exists only in the debug log::

     --- Logging error ---
     Traceback (most recent call last):
       File "/usr/lib/python3.9/logging/__init__.py", line 1086, in emit
	 stream.write(msg + self.terminator)
     UnicodeEncodeError: 'ascii' codec can't encode characters in position 79-89: ordinal not in range(128)
     Call stack:
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/flask/app.py", line 2464, in __call__
	 return self.wsgi_app(environ, start_response)
       File "/usr/local/searx/searx-src/searx/webapp.py", line 1316, in __call__
	 return self.app(environ, start_response)
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/werkzeug/middleware/proxy_fix.py", line 169, in __call__
	 return self.app(environ, start_response)
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/flask/app.py", line 2447, in wsgi_app
	 response = self.full_dispatch_request()
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/flask/app.py", line 1950, in full_dispatch_request
	 rv = self.dispatch_request()
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/flask/app.py", line 1936, in dispatch_request
	 return self.view_functions[rule.endpoint](**req.view_args)
       File "/usr/local/searx/searx-src/searx/webapp.py", line 766, in search
	 number_of_results=format_decimal(number_of_results),
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/flask_babel/__init__.py", line 458, in format_decimal
	 locale = get_locale()
       File "/usr/local/searx/searx-pyenv/lib/python3.9/site-packages/flask_babel/__init__.py", line 226, in get_locale
	 rv = babel.locale_selector_func()
       File "/usr/local/searx/searx-src/searx/webapp.py", line 249, in get_locale
	 logger.debug("%s uses locale `%s` from %s", request.url, locale, locale_source)
     Unable to print the message and arguments - possible formatting error.
     Use the traceback above to help find the error.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>