[mod] upgrade requests to version 2.24.0. use ssl instead of pyopenssl.

requests 2.24.0 uses the ssl module except if it doesn't support SNI, in this case searx fallbacks to pyopenssl.
searx logs a critical message and exit if the ssl modules doesn't support SNI and pyOpenSSL is not installed.
searx logs a critical message and exit if the ssl version is older than 1.0.2.
in requirements.txt, pyopenssl is still required to install searx as a fallback.