Make more tests run on Windows (#8446)

* Remove worker Wait() call due to SIGCHLD being ignored

* Port _pid_alive to Windows

* Show PID as well as TID in glog

* Update TensorFlow version for Python 3.8 on Windows

* Handle missing Pillow on Windows

* Work around dm-tree PermissionError on Windows

* Fix some lint errors on Windows with Python 3.8

* Simplify torch requirements

* Quiet git clean

* Handle finalizer issues

* Exit with the signal number

* Get rid of wget

* Fix some Windows compatibility issues with tests

Co-authored-by: Mehrdad <noreply@github.com>

python/ray/worker.py

@@ -9,7 +9,6 @@ import json
 import logging
 import os
 import redis
-import signal
 from six.moves import queue
 import sys
 import threading
@@ -900,7 +899,7 @@ atexit.register(shutdown, True)
 
 # TODO(edoakes): this should only be set in the driver.
 def sigterm_handler(signum, frame):
-    sys.exit(signal.SIGTERM)
+    sys.exit(signum)
 
 
 try:
@@ -1366,7 +1365,12 @@ def disconnect(exiting_interpreter=False):
     worker.node = None  # Disconnect the worker from the node.
     worker.cached_functions_to_run = []
     worker.serialization_context_map.clear()
-    ray.actor.ActorClassMethodMetadata.reset_cache()
+    try:
+        ray_actor = ray.actor
+    except AttributeError:
+        ray_actor = None  # This can occur during program termination
+    if ray_actor is not None:
+        ray_actor.ActorClassMethodMetadata.reset_cache()
 
 
 @contextmanager