Include psutil module in the python build to enable emrun to track when browser process finishes. (#1464)

Without this, emrun web server will be left running after user code
quits the page abnormally without calling `exit()` (e.g. due to a page
awwsnap crash), and there are multiple processes in use by the browser.

emrun has a graceful fallback to weaker browser process detection when
psutil is not available, so this is easy to miss if not running emrun
with --verbose:
https://github.com/emscripten-core/emscripten/blob/814ec05f74eac9025c0442f51fa09bd8d02f8b43/emrun.py#L330-L339

scripts/update_python.py

diff --git a/scripts/update_python.py b/scripts/update_python.py
index b39597d..a5848ea 100755
--- a/scripts/update_python.py
+++ b/scripts/update_python.py
@@ -148,13 +148,17 @@
 
     install_dir = os.path.join(src_dir, 'install')
 
-    # Install requests module.  This is needed in particualr on macOS to ensure
+    # Install requests module.  This is needed in particular on macOS to ensure
     # SSL certificates are available (certifi in installed and used by requests).
     pybin = os.path.join(src_dir, 'install', 'usr', 'local', 'bin', 'python3')
     pip = os.path.join(src_dir, 'install', 'usr', 'local', 'bin', 'pip3')
     check_call([pybin, '-m', 'ensurepip', '--upgrade'])
     check_call([pybin, pip, 'install', 'requests'])
 
+    # Install psutil module. This is needed by emrun to track when browser
+    # process quits.
+    check_call([pybin, pip, 'install', 'psutil'])
+
     dirname = 'python-%s-%s' % (version, revision)
     if os.path.isdir(dirname):
         print('Erasing old build directory ' + dirname)