This makes methods like _replace available in real code, and caught the
bug where we can't use @functools.lru_cache with Transaction arguments,
because they're unhashable due to their mutable members.
For now, this is basically the Python version of
ledger-tag-convert.plx. It knows how to create RT web links from
ticket and attachment IDs. It confirms that those objects actually
exist too. It may grow to encompass other functionality in the
future.
This loads settings from the same environment variables and ~/.rtrc
file as the rt CLI.
Note that it does *not* support RTCONFIG and the config file
searching, because right now that seems like more work for more
trouble to me.
This is the simplest version of a common validation we're going to do:
make sure that a particular piece of metadata has one of a set of
values.
This checker needs some bounds checking but I wanted to err on the
side of committing this early because it introduces so much base
infrastructure.
