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>

doc/develop/py_testing.rst

@@ -69,19 +69,19 @@ To create disk images we have helper functions located in
 `test/py/tests/fs_helper.py` which shall be used in any tests that require
 creating disk images.
 
-Using `virtualenv` to provide requirements
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Using a Python sandbox to provide requirements
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 The recommended way to run the test suite, in order to ensure reproducibility
-is to use `virtualenv` to set up the necessary environment.  This can be done
-via the following commands:
+is to use a Python sandbox such as `python -m venv` to set up the necessary
+environment.  This can be done via the following commands:
 
 
 .. code-block:: console
 
     $ cd /path/to/u-boot
-    $ sudo apt-get install python3 python3-virtualenv
-    $ virtualenv -p /usr/bin/python3 venv
+    $ sudo apt-get install python3 python3-venv
+    $ python3 -m venv venv
     $ . ./venv/bin/activate
     $ pip install -r test/py/requirements.txt