fix: performance regression when sniffio not installed

[CedricCabessa]

discussion: #1102

sniffio is optional and lazily imported in current_async_library, when absent we use asyncio

However the cost of import failure is paid every time the function is called.

This patch moves the import at module level, like it is already done for anyio or trio

Why it happens now?:

Httpcore never explicitly declared sniffio but it was installed via anyio until they stopped depending on it:
agronholm/anyio#1021

Here is a sample to reproduce the issue

import asyncio
import time

import httpcore

REQUESTS = 500
RESPONSE = b"HTTP/1.1 200 OK\r\nContent-Length: 2\r\n\r\nok"

async def handle(reader, writer):
    try:
        while await reader.readuntil(b"\r\n\r\n"):
            writer.write(RESPONSE)
            await writer.drain()
    except (asyncio.IncompleteReadError, ConnectionResetError):
        pass
    writer.close()

async def main():
    try:
        import sniffio  # noqa: F401

        status = "INSTALLED"
    except ImportError:
        status = "ABSENT"

    server = await asyncio.start_server(handle, "127.0.0.1", 0)
    url = f"http://127.0.0.1:{server.sockets[0].getsockname()[1]}/"

    async with httpcore.AsyncConnectionPool() as pool:
        await pool.request("GET", url)  # warm up the pool

        cpu = time.process_time()
        for _ in range(REQUESTS):
            await pool.request("GET", url)
        cpu = time.process_time() - cpu

    print(f"sniffio {status}: {cpu / REQUESTS * 1e6:.0f} us CPU per request")
    server.close()

asyncio.run(main())

On my machine:

httpcore	sniffio	CPU / req
master	installed	~330 µs
master	missing	~500 µs
patched	missing	~330 µs
patched	installed	~330 µs