Added deep and light sleep functions

[makermelissa]

Fixes #20. This adds the deep and light sleep functions.

[dhalbert]

Later, it will also be possible to wake up on button pushes (or pin changes). So you might think about how this will fit into the proposed API. The time arguments could become optional, and there would also be optional button or pin arguments. You may want to make the time arguments require a keyword in that case.

[makermelissa]

Thanks Dan. We can make the time parameter optional once we have the button press wake feature added. I think it will work as it is for now.

[dhalbert]

Thanks for your patience. This is OK by me now. @tannewt any further comments?

[makermelissa]

Just for completeness, here's the test script I used to test neopixels for light sleep:

import time
from adafruit_magtag.magtag import MagTag

magtag = MagTag()
magtag.add_text()
magtag.set_text("Hello")
magtag.peripherals.neopixels.fill(0xFF0000)
time.sleep(2)

magtag.set_text("Going to sleep")
# Exit program and restart in 10 seconds.
magtag.enter_light_sleep(10)

magtag.set_text("I'm awake")
time.sleep(5)

For deep sleep, I basically used a slightly modified version of Dan's script at adafruit/circuitpython#3767 (comment)

[ladyada]

i like!

[tannewt]

Looks good to me too! Thanks!

[makermelissa]

Excellent :)