Mach: introduce Pyrefly for Python type checking, starting with the wpt folder (#37953)

This is the first stage of adopting Pyrefly. It introduces the Python
folder and focuses on fixing issues around it.

Testing: *Describe how this pull request is tested or why it doesn't
require tests*
Fixes: *Link to an issue this pull requests fixes or remove this line if
there is no issue*

---------

Signed-off-by: Jerens Lensun <jerensslensun@gmail.com>

python/wpt/__init__.py

@@ -7,6 +7,7 @@
 # option. This file may not be copied, modified, or distributed
 # except according to those terms.
 
+from argparse import ArgumentParser
 import os
 import sys
 
@@ -25,7 +26,7 @@ import localpaths  # noqa: F401,E402
 import wptrunner.wptcommandline  # noqa: E402
 
 
-def create_parser():
+def create_parser() -> ArgumentParser:
     parser = wptrunner.wptcommandline.create_parser()
     parser.add_argument(
         "--rr-chaos", default=False, action="store_true", help="Run under chaos mode in rr until a failure is captured"
@@ -60,5 +61,5 @@ def create_parser():
     return parser
 
 
-def run_tests():
+def run_tests() -> bool:
     return test.run_tests()