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/tests/test_autoscaler.py

@@ -1,3 +1,4 @@
+import os
 import shutil
 import tempfile
 import threading
@@ -165,7 +166,7 @@ SMALL_CLUSTER = {
     },
     "auth": {
         "ssh_user": "ubuntu",
-        "ssh_private_key": "/dev/null",
+        "ssh_private_key": os.devnull,
     },
     "head_node": {
         "TestProp": 1,
@@ -310,7 +311,7 @@ class AutoscalingTest(unittest.TestCase):
         return path
 
     def testInvalidConfig(self):
-        invalid_config = "/dev/null"
+        invalid_config = os.devnull
         with pytest.raises(ValueError):
             StandardAutoscaler(
                 invalid_config, LoadMetrics(), update_interval_s=0)