bootstrap: Adding more output when installing dependencies (#31003)

It's often the case (especially with the taplo installation and on
Windows) that bootstrap is doing lots of stuff in the background for a
long amount of time. Without output it's hard to tell what exactly is
going on. This change adds more output to this process as well as
removing some Pythong 2.x era code.

python/mach_bootstrap.py

@@ -5,12 +5,9 @@
 import os
 import platform
 import site
-import shutil
+import subprocess
 import sys
 
-from subprocess import Popen
-from tempfile import TemporaryFile
-
 SCRIPT_PATH = os.path.abspath(os.path.dirname(__file__))
 TOP_DIR = os.path.abspath(os.path.join(SCRIPT_PATH, ".."))
 WPT_PATH = os.path.join(TOP_DIR, "tests", "wpt")
@@ -103,31 +100,12 @@ def _get_virtualenv_lib_dir():
 
 
 def _process_exec(args):
-    with TemporaryFile() as out:
-        with TemporaryFile() as err:
-            process = Popen(args, stdout=out, stderr=err)
-            process.wait()
-            if process.returncode:
-                print('"%s" failed with error code %d:' % ('" "'.join(args), process.returncode))
-
-                if sys.version_info >= (3, 0):
-                    stdout = sys.stdout.buffer
-                else:
-                    stdout = sys.stdout
-
-                print('Output:')
-                out.seek(0)
-                stdout.flush()
-                shutil.copyfileobj(out, stdout)
-                stdout.flush()
-
-                print('Error:')
-                err.seek(0)
-                stdout.flush()
-                shutil.copyfileobj(err, stdout)
-                stdout.flush()
-
-                sys.exit(1)
+    try:
+        subprocess.check_output(args, stderr=subprocess.STDOUT)
+    except subprocess.CalledProcessError as exception:
+        print(exception.output.decode(sys.stdout.encoding))
+        print(f"Process failed with return code: {exception.returncode}")
+        sys.exit(1)
 
 
 def _activate_virtualenv(topdir):
@@ -137,6 +115,7 @@ def _activate_virtualenv(topdir):
     if os.environ.get("VIRTUAL_ENV") != virtualenv_path:
         venv_script_path = os.path.join(virtualenv_path, _get_virtualenv_script_dir())
         if not os.path.exists(virtualenv_path):
+            print(" * Setting up virtual environment...")
             _process_exec([python, "-m", "venv", "--system-site-packages", virtualenv_path])
 
         # This general approach is taken from virtualenv's `activate_this.py`.
@@ -178,6 +157,7 @@ def _activate_virtualenv(topdir):
         except OSError:
             pass
 
+        print(f" * Installing Python requirements from {req_path}...")
         _process_exec([python, "-m", "pip", "install", "-I", "-r", req_path])
 
         open(marker_path, 'w').close()