Handle `NotImplementedError: unsupported platform` case.

[tomchristie]

If these three things occur...

• The user is using httpcore either with standard sync, or with asyncio.
• The trio package happens to be installed.
• We are running on a platform that's unsupported by Trio.

Then there's a case where an import of trio can occur and trigger a NotImplementedError: unsupported platform exception. (python-trio/trio#3013)

I'd suggest:

• We start with a nice simple fix, and catch the case here...

  httpcore/httpcore/_synchronization.py

  Line 12 in ba3f942

  except ImportError: # pragma: nocover

• Ideally we ensure that we don't run import trio or import anyio unless we're actually running async.