doc: pytest: mention additional requirements for venv

doc: pytest: mention additional requirements for venv

Add a paragraph explaining that in addition to the requirements.txt
for test/py/test.py itself, users may need to install additional python
packages depending on the U-Boot configuration being built.

Reviewed-by: Simon Glass <simon.glass@canonical.com>
Tested-by: Mattijs Korpershoek <mkorpershoek@kernel.org> # sandbox
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>