Aidegen: add ver requirement for running on MacOS.

Darwin requires python3 to run aidegen properly. This patch
is to provide useful information without installing python for users.

Test: (for Darwin only)
    $ #cherry-pick aosp/857951
    $ #cherry-pick aosp/853111
    $ source build/envsetup.sh; lunch
    $ aidegen -v tradefed
Change-Id: I14c4bb2370e9c90f653e0078da8891d633eaaba3