caltechlibrary/handprint: Release 1.5.1

There are no functional changes in this release; all of the code changes are internal, to speed up Handprint startup times and further clean up some code dependencies. All changes in this release: * To speed up initial application startup time, `import` statements have been moved from (usually) the tops of files, to where the packages are actually used in the code. Profiling using the Python `-X importtime` option helped identify expensive packages to make this efficiency gain possible. * The user interface code in `ui.py` has been replaced with importing [Bun](https://github.com/caltechlibrary/bun). * The version of `commonpy` needed is now 1.3.9. * The dependency on [dateparser](https://github.com/scrapinghub/dateparser) has been removed because Handprint doesn't depend on that package. (It's a transitive dependency coming from something else, but by itself, it doesn't belong in Handprint's requirements.)