Fix non-ASCII (Korean) repo backup failure

[GabrielDougherty]

Description

This PR fixes an issue where backing up to a repository with a non-ASCII (Korean) name failed due to incorrect length checking logic on MacOS. I updated the code to do .encode() to pass the length in bytes, rather than the simple character length, since Korean characters are multi-byte and since the Apple function SecKeychainFindGenericPassword appears to use byte length. The docs are ambiguous on whether they use byte length:

serviceNameLength
The length of the serviceName character string.

but my change is consistent with the behavior of C strlen() UTF-8 handling, and it fixes the issue.

Related Issue

#2332

Motivation and Context

Backups weren't working when using a repository with a Korean name.

How Has This Been Tested?

I ran through the reproduction steps in the linked issue with my change and observed that the backup worked, while it failed without the change.

Screenshots (if appropriate):

Types of changes

• 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 change)

Checklist:

• I have read the CONTRIBUTING guide.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

I provide my contribution under the terms of the license of this repository and I affirm the Developer Certificate of Origin.

[GabrielDougherty]

I think I've fixed the test failure, by doing a conditional import for the MacOS-specific module. 🤞

[m3nu]

Thanks for the useful addition!