src/sage/doctest/util.py: support macOS if psutil is installed

If psutil is installed (it usually will be, because ipython uses it),
we can use it to compute the cputime used by pexpect processes on
macOS. This is of course guarded by a try/except clause, and may not
always work, but is better than giving up immediately on macOS.

src/sage/doctest/util.py

@@ -289,14 +289,25 @@ def _quick_cputime(self, expect_objects):
             S = s()
             if S and S.is_running():
                 try:
+                    # This will fail anywhere but linux/BSD, but
+                    # there's no good cross-platform way to get the
+                    # cputimes from pexpect interfaces without totally
+                    # mucking up the doctests.
                     path = f"/proc/{S.pid()}/stat"
                     cputime += self._proc_stat_cpu_seconds(path)
                 except OSError:
-                    # This will fail anywhere but linux/BSD, but
-                    # there's no good cross-platform way to get the
-                    # cputimes from pexpect interfaces without
-                    # totally mucking up the doctests.
-                    pass
+                    # If we're on macOS, we can fall back to using
+                    # psutil, but only if it's installed. It's usually
+                    # installed as a transitive dependency (ipython
+                    # needs it), but it isn't explicitly listed as
+                    # a dependency of sagelib.
+                    try:
+                        from psutil import Process
+                        cputime += sum(Process(S.pid()).cpu_times()[0:2])
+                    except (ImportError, ValueError):
+                        # ImportError: no psutil
+                        # ValueError: invalid (e.g. negative) PID
+                        pass
 
         return cputime