Support httpx 0.21.1 and httpcore 0.14.3

[HLFH]

httpcore 0.14 got a redesign.
See this as well.

After updating them, I am currently getting the issue:

déc. 07 15:37:46 arch-server uwsgi[338276]: AttributeError: module 'httpcore._utils' has no attribute 'URL'
déc. 07 15:37:46 arch-server uwsgi[338276]: unable to load app 0 (mountpoint='') (callable not found or import error)

In https://github.com/searxng/searxng/blob/master/searx/network/client.py, this would not work:

async def close_connections_for_url(
    connection_pool: httpcore.AsyncConnectionPool, url: httpcore._utils.URL
):

[dalf]

According to the last commits I've seen on httpx, close_connections_for_url can be removed (connections are now closed even in case of error).

I think AsyncProxyTransportFixed and AsyncHTTPTransportFixed can be more simple (untested):

class AsyncProxyTransportFixed(AsyncProxyTransport):
    async def handle_async_request(
        self, method, url, headers=None, stream=None, extensions=None
    ):
		try:
			return await super().handle_async_request(
				method, url, headers=headers, stream=stream, extensions=extensions
			)
		except (ProxyConnectionError, ProxyTimeoutError, ProxyError) as e:
			raise httpx.ProxyError from e
		except OSError as e:
			# socket.gaierror when DNS resolution fails
			raise httpx.ConnectError from e


class AsyncHTTPTransportFixed(httpx.AsyncHTTPTransport):
    async def handle_async_request(
        self, method, url, headers=None, stream=None, extensions=None
    ):
		try:
			return await super().handle_async_request(
				method, url, headers=headers, stream=stream, extensions=extensions
			)
		except OSError as e:
			# socket.gaierror when DNS resolution fails
			raise httpx.ConnectError from e

---

I will give a try by the end of the week.

[HLFH]

Closing in favour of #596.