[fix] engine & network issues / documentation and type annotations

This patch fixes some quirks and issues related to the engines and the network.
Each engine has its own network and this network was broken for the following
engines[1]:

- archlinux
- bing
- dailymotion
- duckduckgo
- google
- peertube
- startpage
- wikipedia

Since the files have been touched anyway, the type annotaions of the engine
modules has also been completed so that error messages from the type checker are
no longer reported.

Related and (partial) fixed issue:

- [1] https://github.com/searxng/searxng/issues/762#issuecomment-1605323861
- [2] https://github.com/searxng/searxng/issues/2513
- [3] https://github.com/searxng/searxng/issues/2515

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

searx/settings_defaults.py

@@ -209,9 +209,7 @@ SCHEMA = {
         'enable_http2': SettingsValue(bool, True),
         'verify': SettingsValue((bool, str), True),
         'max_request_timeout': SettingsValue((None, numbers.Real), None),
-        # Magic number kept from previous code
         'pool_connections': SettingsValue(int, 100),
-        # Picked from constructor
         'pool_maxsize': SettingsValue(int, 10),
         'keepalive_expiry': SettingsValue(numbers.Real, 5.0),
         # default maximum redirect