qemu_v8: use Python 3 when building EDK2

Set PYTHON3_ENABLE=TRUE so that EDK2 is built with Python 3. Python 2 is
deprecated. Requires EDK2 tag edk2-stable201905 or later (older versions
simply ignore PYTHON3_ENABLE).

Signed-off-by: Jerome Forissier <jerome@forissier.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>