httpx.AsyncClient报错ProxyError: 504 Gateway Time-out

## 场景：

同一个URL，用requests.get能正常获取网页内容，用httpx.get和httpx.AsyncClient.get就不行，要么就报超时，要么就报ProxyError: 504 Gateway Time-out

import anyio, httpx, requests

url = 'https://my-domain.com/xxx/'

r = requests.get(url, verify=False) # 这个ok
r = httpx.get(url, verify=False) # 这个报错了

async def fetch(url):
    async with httpx.AsyncClient(verify=False) as client:
        return await client.get(url)

r = anyio.run(fetch, url) # 这个也报错了

原因：httpx在Windows下，默认读取系统的代理设置，然后它却不能读到跳过代理的白名单，于是导致不该走代理的网址走了代理，从而引发代理错误。

解决：加上trust_env=False即可

import anyio, httpx, requests

url = 'https://my-domain.com/xxx/'

r = httpx.get(url, verify=False, trust_env=False)

async def fetch(url):
    async with httpx.AsyncClient(verify=False, trust_env=False) as client:
        return await client.get(url)

r = anyio.run(fetch, url)