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)

test/test_threads.py

@@ -65,7 +65,7 @@ def test_using_threads():
     capnp.remove_event_loop(True)
     capnp.create_event_loop(True)
 
-    read, write = socket.socketpair(socket.AF_UNIX)
+    read, write = socket.socketpair()
 
     def run_server():
         _ = capnp.TwoPartyServer(write, bootstrap=Server())