scons: bump required python version to 2.7 to support pybind11
scons: bump required python version to 2.7 to support pybind11

Change-Id: Ic3652f975477f2e5d144e054489ab73ed9f82b55
Reviewed-on: https://gem5-review.googlesource.com/4440
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Reviewed-by: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Reviewed-by: Joe Gross <15c33eed775d43fab32b86e60cf53409dcf8e1dc@amd.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>

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