bpo-40275: test.support imports subprocess lazily (GH-20471)

test.support module now imports the platform and subprocess modules
lazily to reduce the number of modules imported by
"import test.support".

With this change, the threading module is no longer imported
indirectly by "import test.support".

Use sys.version rather than platform.machine() to detect the Windows
ARM32 buildbot.

Lib/test/support/__init__.py

@@ -12,11 +12,9 @@
 import importlib
 import importlib.util
 import os
-import platform
 import re
 import stat
 import struct
-import subprocess
 import sys
 import sysconfig
 import time
@@ -81,7 +79,7 @@
 # The timeout should be long enough for connect(), recv() and send() methods
 # of socket.socket.
 LOOPBACK_TIMEOUT = 5.0
-if sys.platform == 'win32' and platform.machine() == 'ARM':
+if sys.platform == 'win32' and ' 32 bit (ARM)' in sys.version:
     # bpo-37553: test_socket.SendfileUsingSendTest is taking longer than 2
     # seconds on Windows ARM32 buildbot
     LOOPBACK_TIMEOUT = 10
@@ -481,6 +479,7 @@ def forget(modname):
 def _is_gui_available():
     if hasattr(_is_gui_available, 'result'):
         return _is_gui_available.result
+    import platform
     reason = None
     if sys.platform.startswith('win') and platform.win32_is_iot():
         reason = "gui is not available on Windows IoT Core"
@@ -581,6 +580,7 @@ def _requires_unix_version(sysname, min_version):
     def decorator(func):
         @functools.wraps(func)
         def wrapper(*args, **kw):
+            import platform
             if platform.system() == sysname:
                 version_txt = platform.release().split('-', 1)[0]
                 try:
@@ -627,6 +627,7 @@ def decorator(func):
         @functools.wraps(func)
         def wrapper(*args, **kw):
             if sys.platform == 'darwin':
+                import platform
                 version_txt = platform.mac_ver()[0]
                 try:
                     version = tuple(map(int, version_txt.split('.')))
@@ -1607,6 +1608,7 @@ def start(self):
             sys.stderr.flush()
             return
 
+        import subprocess
         with f:
             watchdog_script = findfile("memory_watchdog.py")
             self.mem_watchdog = subprocess.Popen([sys.executable, watchdog_script],
@@ -2088,11 +2090,13 @@ def swap_item(obj, item, new_val):
 def args_from_interpreter_flags():
     """Return a list of command-line arguments reproducing the current
     settings in sys.flags and sys.warnoptions."""
+    import subprocess
     return subprocess._args_from_interpreter_flags()
 
 def optim_args_from_interpreter_flags():
     """Return a list of command-line arguments reproducing the current
     optimization settings in sys.flags."""
+    import subprocess
     return subprocess._optim_args_from_interpreter_flags()
 
 
@@ -2233,6 +2237,7 @@ def __exit__(self, exc_type, exc_value, exc_tb):
                     print("failed to clean up {}: {}".format(link, ex))
 
     def _call(self, python, args, env, returncode):
+        import subprocess
         cmd = [python, *args]
         p = subprocess.Popen(cmd, stdout=subprocess.PIPE,
                              stderr=subprocess.PIPE, env=env)
@@ -2261,6 +2266,7 @@ def can_xattr():
     if not hasattr(os, "setxattr"):
         can = False
     else:
+        import platform
         tmp_dir = tempfile.mkdtemp()
         tmp_fp, tmp_name = tempfile.mkstemp(dir=tmp_dir)
         try:
@@ -2445,6 +2451,7 @@ def __enter__(self):
                     pass
 
             if sys.platform == 'darwin':
+                import subprocess
                 # Check if the 'Crash Reporter' on OSX was configured
                 # in 'Developer' mode and warn that it will get triggered
                 # when it is.
@@ -2591,6 +2598,7 @@ def setswitchinterval(interval):
     if is_android and interval < minimum_interval:
         global _is_android_emulator
         if _is_android_emulator is None:
+            import subprocess
             _is_android_emulator = (subprocess.check_output(
                                ['getprop', 'ro.kernel.qemu']).strip() == b'1')
         if _is_android_emulator: