Migrate from pendulum to arrow for dates

[pronovic]

The latest implementation of pendulum is partially written in Rust. I've repeatedly had problems getting it to install properly as part of my Poetry development environment — especially when using Python 3.13 on Windows 10 and my M3 Macbook. Depending on the platform, there is apparently not always a binary wheel available, and then Poetry has to fall back on building one. That depends on a working Rust toolchain, which I typically don't have. I assume this might also be a problem for others who use smartapp-sdk as a dependency. Given the very minimal usage of pendulum in this codebase, I've decided to remove it in favor of arrow.

Note: This is unfortunately a breaking change, since dates are exposed on the interface for various public functions, methods, and instance variables. As a result, I bumped the version to v0.7.0. Because the previous major version was zero, we are subject to the SemVer exception, and it is legitimate to introduce breaking changes between 0.6.9 and 0.7.0. The old and new objects (arrow's Arrow and pendulum's DateTime) have quite similar interfaces, and most clients can probably upgrade with no problems.