[rh:requests] Add handler for requests HTTP library (#3668)

Adds support for HTTPS proxies and persistent connections (keep-alive)

Closes https://github.com/yt-dlp/yt-dlp/issues/1890
Resolves https://github.com/yt-dlp/yt-dlp/issues/4070
Resolves https://github.com/ytdl-org/youtube-dl/issues/32549
Resolves https://github.com/ytdl-org/youtube-dl/issues/14523
Resolves https://github.com/ytdl-org/youtube-dl/issues/13734

Authored by: coletdjnz, Grub4K, bashonly

yt_dlp/YoutubeDL.py

@@ -3968,7 +3968,7 @@ class YoutubeDL:
         })) or 'none'))
 
         write_debug(f'Proxy map: {self.proxies}')
-        # write_debug(f'Request Handlers: {", ".join(rh.RH_NAME for rh in self._request_director.handlers.values())}')
+        write_debug(f'Request Handlers: {", ".join(rh.RH_NAME for rh in self._request_director.handlers.values())}')
         for plugin_type, plugins in {'Extractor': plugin_ies, 'Post-Processor': plugin_pps}.items():
             display_list = ['%s%s' % (
                 klass.__name__, '' if klass.__name__ == name else f' as {name}')
@@ -4057,6 +4057,9 @@ class YoutubeDL:
                     raise RequestError(
                         'file:// URLs are disabled by default in yt-dlp for security reasons. '
                         'Use --enable-file-urls to enable at your own risk.', cause=ue) from ue
+                if 'unsupported proxy type: "https"' in ue.msg.lower():
+                    raise RequestError(
+                        'To use an HTTPS proxy for this request, one of the following dependencies needs to be installed: requests')
             raise
         except SSLError as e:
             if 'UNSAFE_LEGACY_RENEGOTIATION_DISABLED' in str(e):
@@ -4099,6 +4102,8 @@ class YoutubeDL:
                 }),
             ))
         director.preferences.update(preferences or [])
+        if 'prefer-legacy-http-handler' in self.params['compat_opts']:
+            director.preferences.add(lambda rh, _: 500 if rh.RH_KEY == 'Urllib' else 0)
         return director
 
     def encode(self, s):