Try to avoid blocking in read().

IPython/utils/_process_win32.py

@@ -23,6 +23,7 b' import time'
 from ctypes import c_int, POINTER
 from ctypes.wintypes import LPCWSTR, HLOCAL
 from subprocess import STDOUT, TimeoutExpired
+from threading import Thread
 
 # our own imports
 from ._process_common import read_no_interrupt, process_handler, arg_split as py_arg_split
@@ -94,20 +95,25 b' def _find_cmd(cmd):'
 def _system_body(p):
     """Callback for _system."""
     enc = DEFAULT_ENCODING
-    print("READING...")
+
+    def stdout_read():
     for line in read_no_interrupt(p.stdout).splitlines():
         line = line.decode(enc, 'replace')
         print(line, file=sys.stdout)
+
+    def stderr_read():
     for line in read_no_interrupt(p.stderr).splitlines():
         line = line.decode(enc, 'replace')
         print(line, file=sys.stderr)
 
+    Thread(target=stdout_read).start()
+    Thread(target=stderr_read).start()
+
     # Wait to finish for returncode. Unfortunately, Python has a bug where
     # wait() isn't interruptible (https://bugs.python.org/issue28168) so poll in
     # a loop instead of just doing `return p.wait()`.
     while True:
         result = p.poll()
-        print("POLLED")
         if result is None:
             time.sleep(0.01)
         else: