Fix/tw priority bug

[Pete-Hamlin]

Description

Hi!

First of all syncall is still awesome, I genuinely couldn't work without it as part of my daily flow.
Also apologies for not really being involved since I threw the caldav PR at you some time ago!

This is a bug I've noticed when using caldav syncall for a while now, but haven't got around to fixing.
Basically tw treats priority as a UDA, so can technically be anything, while the caldav side expects it to be either L/M/H (the defaults), otherwise it throws an error.

As fully supporting UDAs is a massive faff, I've added a simple check that if priority is present in the TW side, if it is not in L/M/H, then it will just be set to 0 on the caldav side. Have also included a test case for the new behaviour.

Let me know if this needs any further info/testing, and thanks once again!

Fixes # (issue)

N/A

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Added pytest case for additional behaviour case
• Have tested on my own home config, and do not get errors anymore

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules