python: Use and refer to the venv module rather than virtualenv

Using some form of sandbox with Python modules is a long standing best
practice with the language. There are a number of ways to have a Python
sandbox be created. At this point in time, it seems the Python community
is moving towards using the "venv" module provided with Python rather
than a separate tool. To match that we make the following changes:

- Refer to a "Python sandbox" rather than virtualenv in comments, etc.
- Install the python3-venv module in our container and not virtualenv.
- In our CI files, invoke "python -m venv" rather than "virtualenv".
- In documentation, tell users to install python3-venv and not
  virtualenv.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/tools/buildman/test.py b/tools/buildman/test.py
index c5feb74..e31e6c7 100644
--- a/tools/buildman/test.py
+++ b/tools/buildman/test.py
@@ -987,7 +987,7 @@
         diff = self.call_make_environment(tchn, full_path=True)[0]
         self.assertEqual({b'LC_ALL': b'C'}, diff)
 
-        # Test that virtualenv is handled correctly
+        # Test that Python sandbox is handled correctly
         tchn.override_toolchain = False
         sys.prefix = '/some/venv'
         env = dict(os.environb)