Type hints don't appear to work properly on Python 3 < 3.6

[arcanebyte]

Running master against the Debian 9.9 image results in the cameo-aphid service not starting properly:

debian@beaglebone:~$ /usr/local/lib/cameo-aphid/profile.py --skip_load_pru_firmware --skip_pin_setup profile.image
Traceback (most recent call last):
File "/usr/local/lib/cameo-aphid/profile.py", line 54, in
import profile_plugins
File "/usr/local/lib/cameo-aphid/profile_plugins.py", line 268
conclusion: bytes
^
SyntaxError: invalid syntax

This PEP leads me to believe we'd need Python3 >= 3.6 for this to work, but I'm not 100% on that -- https://www.python.org/dev/peps/pep-0526/.

The Debian 9.9 image includes Python 3.5.3, while the 10.0 image includes Python 3.7.3.

Food for thought! Thanks for your hard work here.