tests: add a check for Python version >= 3.6
tests: add a check for Python version >= 3.6

This is required by some of the strong typing features added in the
Python style check PR.

Signed-off-by: Radu Weiss <e20b2d982c117e9298ee475ba7241e3f2127299b@amazon.com>

firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker,firecracker-microvm/firecracker