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)

capnp/lib/capnp.pyx

@@ -3224,7 +3224,13 @@ cdef class SchemaParser:
 
         module._parser = parser
 
-        fileSchema = parser._parse_disk_file(display_name, file_name, imports)
+        # Some systems (Windows running pytest) add non-directories to the sys.path used for imports
+        # Filter these out so kj doesn't implode when searching paths
+        filtered_imports = []
+        for imp in imports:
+            if _os.path.isdir(imp):
+                filtered_imports.append(imp)
+        fileSchema = parser._parse_disk_file(display_name, file_name, filtered_imports)
         _load(fileSchema, module)
 
         abs_path = _os.path.abspath(file_name)