Zowie

This release fixes issues 13 and 18, updates the required version of some dependencies, and changes the minimum version of Python to version 3.8.

Detailed changes:
* Fix issue 13: filtering by date using the `-d` option is broken.
* Fix issue 18: installations using `pipx` failed.
* Updated versions of dependencies in `requirements.txt`.
* Added `requirements-dev.txt`.
* Improved `codemeta.json`.
* Updated `Makefile` and `setup.py` to use versions developed for other projects.
* Updated copyright year in various files.

* Add new `-s` option. Please see the section on [special-case behavior](https://github.com/mhucka/zowie#special-case-behavior) for an explanation.
* Fix issue 9: version incompatibility with Sidetrack version 2.0 package.
* Use lazy imports of Python packages for faster application startup.
* Add script to create zipapp version of Zowie.
* Overhaul Makefile.
* Update versions of all Python packages dependencies, and make `requirements.txt` pin the exact version of the packages used at release time.
* Update `LICENSE` file, which had the wrong license text!

* Add missing dependency to `requirements.txt`
* Mention Zowie can't work with Zotero installations that use the linked attachment file method.

* Fix for issue 4, caused by not checking that a path has been provided on the command line.

* Zowie now operates on all types of files by default, not just PDF files.
* Zowie has a new command-line option: `-f`, to allow you to filter files by their extensions and limit its operation to specific file types instead of all files, if you wish.
* A ready-to-run binary executable for macOS is now available for downloading from GitHub.

This update limits the parts of the [PyObjC](https://pypi.org/project/pyobjc/) package that are required in `requirements.txt`, to improve installation time as well as avoid a conflict between `pyobjc-framework-PubSub` and the Python [PyPubSub](https://pypi.org/project/PyPubSub/) package used by [Bun](https://pypi.org/project/bun/).