configs/orangepi_pc: use Python 3.x for U-Boot build

New U-Boot versions (such as 2020.07 being used) need Python 3.x for
pylibfdt.

Fixes:
 - https://gitlab.com/buildroot.org/buildroot/-/jobs/831421100

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

configs/orangepi_pc_defconfig

@@ -23,6 +23,7 @@ BR2_TARGET_UBOOT_CUSTOM_VERSION=y
 BR2_TARGET_UBOOT_CUSTOM_VERSION_VALUE="2020.07"
 BR2_TARGET_UBOOT_BOARD_DEFCONFIG="orangepi_pc"
 BR2_TARGET_UBOOT_NEEDS_DTC=y
+BR2_TARGET_UBOOT_NEEDS_PYTHON3=y
 BR2_TARGET_UBOOT_NEEDS_PYLIBFDT=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM=y
 BR2_TARGET_UBOOT_FORMAT_CUSTOM_NAME="u-boot-sunxi-with-spl.bin"