Use " quotes instead of ' not to pass the \ to pytest -k
    
    The parser of -k expressions was refactored in pytest 6 and \s no longer work.
    
    The error with pytest 6.0.1:
    
    ERROR: Wrong expression passed to '-k': not test_unknown_server \
        and not test_socks5_auth and not \
        test_server_not_found_error_is_raised_for_invalid_hostname and not \
        test_functional_noproxy_star_https and not \
        test_sni_set_servername_callback and not test_not_trusted_ca and not \
        test_invalid_ca_certs_path and not test_max_tls_version and not \
        test_get_301_via_https and not test_client_cert_password_verified and not\
        test_get_via_https and not test_min_tls_version and not\
        test_client_cert_verified: at column 25: unexpected character "\"