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_server.py

@@ -36,7 +36,6 @@ async def mywriter(server, writer):
     while True:
         data = await server.read(4096)
         writer.write(data.tobytes())
-        await writer.drain()
 
 
 async def myserver(reader, writer):