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>

tools/u_boot_pylib/test_util.py

@@ -64,7 +64,7 @@ def run_test_coverage(prog, filter_fname, exclude_list, build_dir,
     if build_dir:
         prefix = 'PYTHONPATH=$PYTHONPATH:%s/sandbox_spl/tools ' % build_dir
 
-    # Detect a Python virtualenv and use 'coverage' instead
+    # Detect a Python sandbox and use 'coverage' instead
     covtool = ('python3-coverage' if sys.prefix == sys.base_prefix else
                'coverage')