Add a --version option

[rdiez]

First of all, many thanks for this great script.

I would like to have a --version option, so that I can quickly check whether the script is out of date, or simply what version I am using now.

I am sure that there is some pip magic to get the installed package version, but I do not know Python at all. And I am guessing that you can install the script with some other means.

[FlashSheridan]

I’d also recommend getting less modest about your branding; rotate-backups deserves having its horn tooted. I installed it yesterday, and was documenting it internally today, and, after a web search, landed on a different tool with a similar name.
As noted above, “/usr/local/bin/rotate-backups --version” didn’t help. “--help” helped, but was overly modest. Even looking at the source of /usr/local/bin/rotate-backups didn’t work. “pip3 search rotate_backups” gave an error that was not your fault. “pip3 show rotate_backups” finally led me back here, but that’s not something my hypothetical successors would necessarily have known.