Raise a neater RuntimeError when the correct async deps are not installed.

[tomchristie]

Closes #825.

example.py

import httpcore
import asyncio


async def main():
    async with httpcore.AsyncConnectionPool() as pool:
        response = await pool.request("GET", "https://www.example.com")
    print(response)


asyncio.run(main())

output:

$ venv/bin/python ./example.py 
Traceback (most recent call last):
  File "/Users/tomchristie/Temp/./example.py", line 11, in <module>
    asyncio.run(main())
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/runners.py", line 44, in run
    return loop.run_until_complete(main)
  File "/Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/asyncio/base_events.py", line 649, in run_until_complete
    return future.result()
  File "/Users/tomchristie/Temp/./example.py", line 6, in main
    async with httpcore.AsyncConnectionPool() as pool:
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_async/connection_pool.py", line 331, in __aenter__
    async with self._pool_lock:
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_synchronization.py", line 64, in __aenter__
    self.setup()
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_synchronization.py", line 56, in setup
    self._backend = current_async_library()
  File "/Users/tomchristie/GitHub/encode/httpcore/httpcore/_synchronization.py", line 35, in current_async_library
    raise RuntimeError(
RuntimeError: Running with asyncio requires installation of 'httpcore[asyncio]'.

Compare this against... #825 (comment)

[chamikabm]

Any plan on releasing this fix ?

[tomchristie]

Any plan on releasing this fix ?

Yes. This PR is currently blocked on an approving review from an @encode/maintainers member.

Either:

• If any of the existing reviewers/commenters on this issue would like I can send an invitation.
• We get a review from an existing member.