Add exceptions for invalid trace callback

[karpetrosyan]

Examples

import httpcore

async def log(a, b):
    print(a, b)

httpcore.request("GET", "https://google.com", extensions={"trace": log})

Old output

RuntimeWarning: coroutine 'log' was never awaited
  self.trace_extension(prefix_and_name, info)
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

New output

TypeError: If you are using a synchronous interface, the callback of the `trace` extension should be a normal function instead of an asynchronous function.

import trio

def log(a, b):
    print(a,b)

async def req():
    cl = httpcore.AsyncConnectionPool()
    await cl.request("GET", "https://google.com", extensions={"trace": log})

trio.run(req)

Old output

TypeError: object NoneType can't be used in 'await' expression

New output

TypeError: If you're using an asynchronous interface, the callback of the `trace` extension should be an asynchronous function rather than a normal function.

[karpetrosyan]

When migrating from synchronous to asynchronous interface, it's common to forget to change the synchronous trace callback to an asynchronous function, so let's add a user-friendly behavior for those cases.

[tomchristie]

Sure.