oe-buildenv-internal: python 3.5 as min version

Python 3.4 is EOL:
https://www.python.org/downloads/release/python-3410/

The last supported distro was probably CentOS-7, which has python36 available
from epel-7 or scl (as rh-python36) [1]

[1] https://www.softwarecollections.org/en/scls/rhscl/rh-python36/



Signed-off-by: Tim Orling <timothy.t.orling@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>