Addcopyfighandler

- Fix [issue when using `Agg` backend](https://github.com/joshburnett/addcopyfighandler/issues/20), which doesn't support get_window_title().

- Fixed [an issue with copying when there are multiple figures](https://github.com/joshburnett/addcopyfighandler/issues/19).

Thanks to rgw5267 for fixing a bug related to the [event handler being registered multiple times](https://github.com/joshburnett/addcopyfighandler/issues/17).

- Made backend checks case-insensitive due to undocumented changes in matplotlib 3.9.

Now with Linux support!
- The figure will be copied to the clipboard as a PNG, regardless of `matplotlib.rcParams['savefig.format']`.
- Requires either Qt or GTK libraries for clipboard interaction. Automatically detects which is being used from `matplotlib.get_backend()`.
- Qt support requires `PyQt5` or `PySide2`.
- GTK support requires `pycairo`, `PyGObject` and `PIL`/`pillow` to be installed.
- While Qt and GTK are both supported, they each have different requirements. As such, those requirements will not be automatically installed along with the `addcopyfighandler` module.

Improved application compatibility on Windows (optionally uses Pillow)
- If Pillow is installed, all non-SVG format specifiers will be overridden and the figure will be copied to the clipboard as a Device-Independant Bitmap.
- If Pillow is not installed, the supported format specifiers are `'png'`, `'jpg'`, `'jpeg'` and `'svg'`. All other format specifiers will be overridden and the figure will be copied to the clipboard as PNG data.

SVG support on Windows
- If `matplotlib.rcParams['savefig.format']` is `'svg'`, the figure will be copied to the clipboard as an SVG.
- Alas, SVG output is not currently supported on Linux. Pull requests that enable SVG support there would be welcomed.

- Fixed an issue where some matplotlib installs will default to TkAgg backend, which this module doesn't support. We now force the backend to switch to Qt5Agg or At4Agg when loading this module, saving the user from having to manually specify one of the Qt backends in every analysis.