Add retries for moisture reading

[caternuson]

Test setup:

Test script:

import time
import board, busio
from adafruit_seesaw.seesaw import Seesaw

i2c = busio.I2C(board.SCL, board.SDA)

ss = Seesaw(i2c, addr=0x36)

start_time = time.time()
count = 0

print("Reading...")

while True:
    count += 1
    if ss.moisture_read() > 4095:
        break
    time.sleep(0.1)

print("DONE")
print("count = {}  time = {}".format(count, time.time()-start_time))

BEFORE ☹️

pi@raspberrypi:~/soil_sensor $ python3 soil_test.py 
Reading...
DONE
count = 223  time = 24.15417194366455
pi@raspberrypi:~/soil_sensor $ python3 soil_test.py 
Reading...
DONE
count = 15  time = 1.5329132080078125
pi@raspberrypi:~/soil_sensor $ python3 soil_test.py 
Reading...
DONE
count = 283  time = 30.68214201927185
pi@raspberrypi:~/soil_sensor $ 

AFTER 😁
(ran for ~15 minutes before <CTRL><C>ing)

pi@raspberrypi:~/soil_sensor $ python3 soil_test.py 
Reading...

Basic reading still works:

pi@raspberrypi:~/soil_sensor $ python3
Python 3.5.3 (default, Sep 27 2018, 17:25:39) 
[GCC 6.3.0 20170516] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import board, busio
>>> from adafruit_seesaw.seesaw import Seesaw
>>> i2c = busio.I2C(board.SCL, board.SDA)
>>> ss = Seesaw(i2c, addr=0x36)
>>> ss.moisture_read()
359
>>> ss.moisture_read()
1015
>>> ss.moisture_read()
362
>>> 

[ladyada]

thanks - please release a new minor semver!

[caternuson]

thanks. done - released 1.3.0.