Fixing Windows tests

- Adding import path filter to exclude non-directories
  Otherwise kj will through exceptions
- Skipped AF_UNIX socket test
- Use default socket configuration when it doesn't matter the type of
socket used
- Open files with utf8 encoding (needed for text validation)
- Explictly call python executable when running external scripts
- Fix path creation to always use os.path.join
- Added timeout to client wait in some tests

- Some broken tests still remain (most likely asyncio related)

examples/async_client.py

@@ -39,7 +39,6 @@ async def mywriter(client, writer):
     while True:
         data = await client.read(4096)
         writer.write(data.tobytes())
-        await writer.drain()
 
 
 async def background(cap):