Docs: Specify the version requirement of Python3

This requirement is coming from the 'imgtool' Python package,
which is used to sign the binaries when BL2 is enabled.

Change-Id: I767939dc393c988801d4799d69d94c1fd9c5f21e
Signed-off-by: David Vincze <david.vincze@linaro.org>
1 file changed