Add support for no_proxy and case insensitive env variables (#9372)

### Changes proposed in this PR

- Add support for the `no_proxy` and `NO_PROXY` environment variables
  - Internally rely on urllib's [`proxy_bypass_environment`](bdb941be42/Lib/urllib/request.py (L2519))
- Extract env variables using urllib's `getproxies`/[`getproxies_environment`](bdb941be42/Lib/urllib/request.py (L2488)) which supports lowercase + uppercase, preferring lowercase, except for `HTTP_PROXY` in a CGI environment

This does contain behaviour changes for consumers so making sure these are called out:
- `no_proxy`/`NO_PROXY` is now respected
- lowercase `https_proxy` is now allowed and taken over `HTTPS_PROXY`

Related to #9306 which also uses `ProxyAgent`

Signed-off-by: Timothy Leung tim95@hotmail.co.uk

synapse/server.py

@@ -24,7 +24,6 @@
 import abc
 import functools
 import logging
-import os
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -370,11 +369,7 @@ class HomeServer(metaclass=abc.ABCMeta):
         """
         An HTTP client that uses configured HTTP(S) proxies.
         """
-        return SimpleHttpClient(
-            self,
-            http_proxy=os.getenvb(b"http_proxy"),
-            https_proxy=os.getenvb(b"HTTPS_PROXY"),
-        )
+        return SimpleHttpClient(self, use_proxy=True)
 
     @cache_in_self
     def get_proxied_blacklisted_http_client(self) -> SimpleHttpClient:
@@ -386,8 +381,7 @@ class HomeServer(metaclass=abc.ABCMeta):
             self,
             ip_whitelist=self.config.ip_range_whitelist,
             ip_blacklist=self.config.ip_range_blacklist,
-            http_proxy=os.getenvb(b"http_proxy"),
-            https_proxy=os.getenvb(b"HTTPS_PROXY"),
+            use_proxy=True,
         )
 
     @cache_in_self