Ensure checksum is 2 digits

[Lukanite]

Fix bug where some L76 messages are ignored due to the checksum being one digit long (i.e. 'F') vs. the expected 2 digit length ('0F')

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What does this implement/fix? Explain your changes.

Sometimes, messages written to the L76 are ignored due to an invalid checksum

Does this close any currently open issues?

None afaik

Any relevant logs, error output, etc?

(If it’s long, please paste to https://gist.github.com and insert the link here)
l76.write("PMTK740,{},{},{},{},{},{}".format(curtime[0], curtime[1], curtime[2], curtime[3], curtime[4], curtime[5])) used to result in
$PMTK740,2021,4,8,19,0,57*6 being sent, which was missing the second checksum digit, causing the message to be ignored

Any other comments?

Where has this been tested?

• Board type and hardware version:
  Pytrack 2.0X, GPY 1.0
• os.uname() output:
  (sysname='GPy', nodename='GPy', release='1.20.2.r4', version='v1.11-ffb0e1c on 2021-01-12', machine='GPy with ESP32', pybytes='1.6.1')

[gijsio]

Thanks for the contribution, I do indeed believe this is a bug in edge-cases where the (insignificant) 0 on the checksum is dropped. The fix looks good to me!

I noticed the 740 packet ID is not included in the default documentation, but can be found in the extended documentation to set the time on the GPS chip to improve the time to first fix if Im correct?

[Lukanite]

Yeah, that's right. Looks like some of the commands from other products just happen to work, so I've been trying 740 (time)/741 (position):
https://cdn.sparkfun.com/assets/b/c/4/4/5/AirPrime_XM_XA_Series_Software_User_Guide_r3.pdf